c:\@Work\Perl\monks>perl -wMstrict -le
"use Data::Dump qw(pp);
 ;;
 my %test = (
   qr{ f [eio]+ e }xms => 'regex 1',
   qr{pure string}     => 'regex 2',
   'pure string'       => 'string 1',
   );
 pp \%test;
 ;;
 for my $str (
   'a pure string it is, yes!',
   'a pure    string too',
   'xxx fiiiie yyy',
   ) {
   print qq{'$str'};
   ;;
   for my $k (keys %test) {
     die $k, ' not a pure string: reference ', ref $k if ref $k;
     printf qq{$test{$k}: $k: };
     print $str =~ /($k)/ ? qq{match, \$1 is '$1'} : 'NO match';
     }
   print '';
   }
"
{
  "(?^:pure string)"     => "regex 2",
  "(?^msx: f [eio]+ e )" => "regex 1",
  "pure string"          => "string 1",
}

'a pure string it is, yes!'
regex 1: (?^msx: f [eio]+ e ): NO match
string 1: pure string: match, $1 is 'pure string'
regex 2: (?^:pure string): match, $1 is 'pure string'

'a pure    string too'
regex 1: (?^msx: f [eio]+ e ): NO match
string 1: pure string: NO match
regex 2: (?^:pure string): NO match

'xxx fiiiie yyy'
regex 1: (?^msx: f [eio]+ e ): match, $1 is 'fiiiie'
string 1: pure string: NO match
regex 2: (?^:pure string): NO match