$/ = \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;
}