$qr_a = qr/\w/; # Pre-compiled during BEGIN {}
$qr_b = qr/\d/; # Pre-compiled during BEGIN {}

# Stringify both $qr_a and $qr_b then compile new regex. The previous pre-compilation is not used
$data =~ m/$qr_a$qr_b/;
# match()
#    regcomp() # Compile the expression
#       regcreset

$re_a = "\\w";
$re_b = "\\d";
$data =~ m/$re_a$re_b/;
# match()
#    regcomp() # Compile the expression
#       regcreset