use strict;
use warnings;

my $pattern = shift;
my $target = 'aBC';
my $regex;
eval "\$regex = qr ($pattern)";
print $regex, "\n";
print $target =~ $regex ? "$pattern Worked!\n" : "$pattern Failed!\n";
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The eval line interpolates the pattern before it is turned into a regex by qr.

Also consider $pattern =~ s/(.*)/qq(qq($1))/ee; as a way of doing the interpolation without using a direct eval. This method can still be poisoned using a carefully crafted regex.