my @a = ("foo");

say  "foo"   =~ m |@a| ? 'Found "foo"' : 'Did not find "foo"';
say  "foo"   =~ m '@a' ? 'Found "foo"' : 'Did not find "foo"';
say  'bar@a' =~ m |@a| ? 'Found "@a"'  : 'Did not find "@a"';
say  'bar@a' =~ m '@a' ? 'Found "@a"'  : 'Did not find "@a"';
__END__
Found "foo"
Did not find "foo"
Did not find "@a"
Found "@a"