... what does this line do? qr{(?:@excl)};

That statement, which by the way is really
    qr{(?x) (?: @excl ) \b};
interpolates the strings that have been assigned to the  @excl array into a pre-compiled  qr// "regex object" (see Regexp Quote-Like Operators). The interpolation is under the control of the  $" special variable (see perlvar), which has been assigned the  '|' string, which is the regex alternation operator. The full effect of the block in which this statement executes can easily be seen:

c:\@Work\Perl>perl -wMstrict -le
"my $rxExcl = do {
     my @excl = qw{ if else 1'b0 end };
     local $\" = q{^|};
     qr{(?x) (?: @excl ) \b};
     };
 print $rxExcl;
"
(?^:(?x) (?: if|else|1'b0|end ) \b)
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