In this case, you don't want a regex, at least directly. I think we've golfed this once, but in general:

my $to_be_matched = 'tab';
my $word = 'bat';
if ( join '', sort split(//, $word) eq 
     join '', sort split(//, $to_be_matched ) ) {
   # Success!
} else {
   # Failure
}
[download]

Update - $ instead of @ on to_be_matched

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