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$ perl -Mre=debug -e'/$_/ for 1..2' 2>&1 | grep Compiling
Compiling REx "1"
...

$ perl -Mre=debug -e'$x="foo"; /$x/ for 1..2' 2>&1 | grep Compiling
Compiling REx "foo"

my $pat = 'id\\d+';

while (<>) {
    print if /$pat/;
}

my $pat = qr/id\d+/;

while (<>) {
    print if /$pat/;
}

my @pats = ( qr/fo*/, qr/ba./, qr/w+3/ );

...
        print if /$pat/;
    }
}

my @handlers = (
    { pattern => qr/.../, callback => sub { ... } },
...
        }
    }
}