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";
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