Maybe this will clarify things for you? (Hint: $a !~~ $b parses as $a !~ (~$b))

c:\test>perl -MO=Deparse
use 5.10.0;
use strict;
use warnings (FATAL => 'all');

my $a = "hello";
my $b = "hello";

print "a matches b\n" if($a ~~ $b);
print "a not matches b\n" if($a !~~ $b);
print "a matches not b\n" if($a ~~! $b);
print "a matches and a half b\n" if($a ~~~ $b);
print "a not matches and a half b\n" if($a !~~~ $b);
print "a double matches b\n" if($a ~~~~ $b);
print "a not double matches b\n" if($a !~~~~ $b);
^Z
sub BEGIN {
    require 5.10.0;
}
BEGIN {${^WARNING_BITS} = "\377\377\377\377\377\377\377\377\377\377\37
+7?"}
use strict 'refs';
BEGIN {
    $^H{'feature_say'} = q(1);
    $^H{'feature_state'} = q(1);
    $^H{'feature_switch'} = q(1);
}
my $a = 'hello';
my $b = 'hello';
print "a matches b\n" if $a ~~ $b;
print "a not matches b\n" if not $a =~ ~$b;
print "a matches not b\n" if $a ~~ !$b;
print "a matches and a half b\n" if $a ~~ ~$b;
print "a not matches and a half b\n" if not $a =~ ~~$b;
print "a double matches b\n" if $a ~~ ~~$b;
print "a not double matches b\n" if not $a =~ ~~~$b;
- syntax OK
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