The $string is probably a Unicode string. It makes a difference:

perl -CO -wE ' for my $s ( "á", do { use utf8; "á" } ) {
                  say $s, $s =~ $_ ? 1 : 0
                      for qr/\xc3\xa1/,
                          qr/\xe1/
               }'
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Output in a utf-8 terminal:

á1
á0
á0
á1
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