Everything is passed by reference in Perl.

I know it's true, but the common ways to handle subroutine arguments sort of make it untrue.

sub fiddle {
    my ($arg) = @_;
    $arg = "waffle";
}

sub faddle {
    my $arg = shift;
    $arg = "waffle";
}

sub muddle {
    $_[0] = "waffle";
}

my $value = "pancake";
print "$value\n"; # Prints "pancake"
fiddle($value);
print "$value\n"; # Prints "pancake"
faddle($value);
print "$value\n"; # Prints "pancake"
muddle($value);
print "$value\n"; # Prints "waffle"
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Shifting and slicing @_ into subroutine lexicals will make copies (which only matters for simple non-reference values). I know, I know. This is obvious stuff I'm talking about. But when somebody hears that everything is passed by reference in Perl, they also need to hear that it's a behavior that often gets bypassed in idiomatic code.