As others have mentioned, /(?<=\t)\w+/ does not include the tab in $& which contains the text that the regular expression matched. Perhaps some code might make it a little clearer:

#!/usr/bin/perl
use strict;

my $string = "Testing \tthis regex";

print $string, "\n";

$string =~ /(?<=\t)\w+/;
my $string1 = $&;

$string =~ /\t\w+/;
my $string2 = $&;

print "/(?<=\\t)\w+/:\n" . $string1 . "\n";
print "/\\t\w+/:\n" . $string2 . "\n";
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This is mostly useful for substitutions but hopefully my code makes it a little clearer as to what is going on. The output is as follows:

Testing         this regex
/(?<=\t)w+/:
this
/\tw+/:
        this
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