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;

##</code><code>##

$/ = \32;
my $pos = tell($fh);
while( <$fh> ) {
    if ($_ =~ /something/) {
        my $cur = tell($fh);
        seek $fh, 0, $pos          or die "Can't seek: $!\n";
        print $fh "something else" or die "Can't print: $!\n";
        seel $fh, 0, $cur;
    }
    $pos = tell($fh) or die "Can't tell: $!\n;
}