If the !exists ( $hash{key} ) is actually a literal in your program (and not user input), it's probably better to implement that with code references: 
my %hash;
my $condition = sub { ! exists $hash{key} };

# and later on use it as
if ($condition->()) {
    print "'key' does not exists in \%hash\n";
}

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Perl 6 - second systems done right

In reply to Re: Dynamic execution of expression by moritz
in thread Dynamic execution of expression by Anonymous Monk

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