this only lasts locally in this sub

# Creates a hash %t that has the same contents as %toys
my %t = %{$_[0]};

# Modifies %t (which is unrelated to %toys)
$t{'leggo'} = "yes";
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[$_[0]{'leggo'} = "yes";] is darned ugly!

Since you're not changing the reference itself (only that to which it refers), you can copy the reference into a more suitably named variable.

my ($toys) = @_;
$toys->{'leggo'} = 1;
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Note that 1 and 0 (or undef) are usually used for boolean. It's not that using "yes" is problematic, it's that "no" isn't false. Formatting outputs (e.g. converting to "yes" and "no") should be done on output.