use strict;
use warnings;

my $file = shift or die "no file given\n";
my $offset = 12345;

open my $fh, '+<', $file or die "Cannot open $file for read/write: $!\n";

seek $fh, 0, $offset or die "Cannot seek to $offset in $file: $!\n";
print $fh "blah blah blah" or die "Cannot print to $file: $!\n";
close $fh;