Actually, there is a difference between $output and ${'output'}:

$output = "global";
{
    my $output = "my";

    print $output;
    print ${'output'};
}
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This little example will first print out "my" then "global". This is because the latter is a symbolic reference to the $output variable - hence it will always refer to the global variable (which is stored in the package stash).

${output} on the other hand is not a symbolic reference, and will still refer to the lexical variable (if one exists).

Autark.