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

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

##</code><code>##

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

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

##</code><code>##

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

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

##</code><code>##

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

while (<>) {
    for my $pat (@pats) {
        print if /$pat/;
    }
}

##</code><code>##

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

for my $line (@lines) {
    for my $handler (@handlers) {
        if (my @params = ($line =~ $handler->{$pattern})) {
            my $func = $handler->{callback};
            $func->(@params) if $func;
        }
    }
}