Text file editing_perl-GFF

[bioman1]

I would like to get the help of computer experts, in editing my GFF file

##dsfsd2
##sdf-sdf sasg 5.6.3
gi34_ex Gen  CDS     161     317     .       +       .       Name=Xm ZAK;created by=User
gi56_ex Gen  CDS     2194    2280    .       +       .       Name=Xm ZAK;created by=User
gi37_ex Gen  CDS     2848    2951    .       +       .       Name=Xm ZAK;created by=User
gi37_ex Gen  CDS     4554    4619    .       +       .       Name=Xm ZAK;created by=User
gi37_ex Gen  CDS     4729    4756    .       +       .       Name=Xm ZAK;created by=User
gi37_ex Gen  extracted region        1       11677   .       +       .       Name=Extracted region from gi|371443185|gb|JH556675.1|;Extracted interval="1960862 -> 1972538"

Note that following from third row, the input has 12 ($1 to $12) columns, tab separated. The last line of the file has 16 ($1 to $16) columns, tab separated. ## lines should be ignored.

I want to take $14 of the last line (interval="1960862) only the number (1960862) add to $4 column (161,2194,2848..4729) i.e (161+1960862=1961023,2194+1960862=1963056..) and to $5 column (317,2280,2951..4756) i.e (317+1960862=1961179,2280+1960862=1963142), ignore the last line.

The output should look like this:

##dsfsd2
##sdf-sdf sasg 5.6.3
gi34_ex Gen  CDS     1961023     1961179     .       +       .       Name=Xm ZAK;created by=User
gi56_ex Gen  CDS     1963056    1963142    .       +       .       Name=Xm ZAK;created by=User
gi37_ex Gen  CDS     1963710    1963813    .       +       .       Name=Xm ZAK;created by=User
gi37_ex Gen  CDS     1965416    1965481    .       +       .       Name=Xm ZAK;created by=User
gi37_ex Gen  CDS     1965591    1965618    .       +       .       Name=Xm ZAK;created by=User
gi37_ex Gen  extracted region        1       11677   .       +       .       Name=Extracted region from gi|371443185|gb|JH556675.1|;Extracted interval="1960862 -> 1972538"

I have used perl script as below.But it has some errors, it outputs the same result

#!/usr/bin/perl
use warnings;
use strict;

open my $IN, '<', '1.in' or die $!;

my $line;
$line = $_ while <$IN>;                 # Remember the last line.
my $last = $.;                          # Remember the number of the last line.

my $interval = (split /\t/, $line)[13]; # Extract the 14th column.
$interval =~ s/[^0-9]+//;               # Keep only the number.

seek $IN, 0, 0;                         # Rewind to the beginning of the input.
$. = 0;                                 # Restart the line counter.
my $start = 1;                          # Flag to skip first lines.
while (<$IN>) {
    my @columns = split /\t/;
    /^##/ or undef $start;              # Unset start if the header is over.
    if (not ($start or $. == $last)) {  # Not header or last line?
        $_ += $interval for @columns[3, 4];
    }
    print join "\t", @columns;
}

Any help in perl is appreciated.