my $rx = 'abc';
my $qr = qr/$rx/;
if ('ABC' =~ /$qr/i) {
    print "ABC matches /abc/i\n"
}
else {
    print "ABC does not match /abc/i\n"
}

##</code><code>##

#!/usr/bin/perl -w
use strict;

my @patterns = ('abc', 'xyz');
my %regexes =  map { $_, qr/$_/} @patterns;

my @strings = ('the alphabet starts with ABC',  
               'the alphabet ends with XYZ'
              );

for my $str(@strings) {
    for ( keys %regexes ) {
        print qq("$str" =~ /$_/i => );
        
        if ($str =~ /$regexes{$_}/i) {
            print "(qr) match\n";
        }
        else {
            print "(qr) no match\n"
        }
    }
}

for my $str(@strings) {
    for ( @patterns ) {
        print qq("$str" =~ /$_/i => );
        
        if ($str =~ /$_/i) {
            print "(pattern) match\n";
        }
        else {
            print "(pattern) no match\n"
        }
    }
}

my $string = <<END;
This text spawns
across multiple 
lines
END

my $pattern = 'multiple .+ lines';
my $regex = qr/$pattern/x;

if ($string =~ /$regex/s) {
    print "dot-matches-all (qr) matches\n";
}
else {
    print "dot-matches-all (qr) does not match\n";
}

if ($string =~ /$pattern/xs) {
    print "dot-matches-all (literal) matches\n";
}
else {
    print "dot-matches-all (literal) does not match\n";
}
__END__
"the alphabet starts with ABC" =~ /abc/i => (qr) no match
"the alphabet starts with ABC" =~ /xyz/i => (qr) no match
"the alphabet ends with XYZ" =~ /abc/i => (qr) no match
"the alphabet ends with XYZ" =~ /xyz/i => (qr) no match
"the alphabet starts with ABC" =~ /abc/i => (pattern) match
"the alphabet starts with ABC" =~ /xyz/i => (pattern) no match
"the alphabet ends with XYZ" =~ /abc/i => (pattern) no match
"the alphabet ends with XYZ" =~ /xyz/i => (pattern) match
dot-matches-all (qr) does not match
dot-matches-all (literal) matches

##</code><code>##

$ perl -e 'for (qw( i x s m )) {print eval "qr/perl/$_", "\n"}'
(?i-xsm:perl)
(?x-ism:perl)
(?s-xim:perl)
(?m-xis:perl)