$ perl -le '
    my $w = qr{[A-Za-z0-9_]};
    print q{With \x{424}:};
    print 3+6 if "\x{424}" =~ /$w/;
    print q{With \x{42}:};
    print 3+6 if "\x{42}" =~ /$w/;
'
With \x{424}:
With \x{42}:
9