Are all 'special variables' ($&, $N (N is integer)) expanded in that way too?
All variables are expanded that way, special or not: 
my $var = '[ab]';
my $string = '12a3434';
print "match" if $string =~ /$var/;

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See also quotemeta and the \Q and \E escape sequences.

"What should it profit a man, if he should win a flame war, yet lose his cool?"

In reply to Re^3: Regex error when [] occurs in file.. by Joost
in thread Regex error when [] occurs in file.. by why_bird

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