my $param = shift;
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$param is a lexical variable so you have no way of getting at it. Make it a global variable and you can get at it from the outside:

our $param = shift;
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... or even better, consider initializing the parameters from @ARGV, and doing so only in main, or passing @ARGV explicitly to main:

say main(@ARGV) unless caller();

sub main {
    my( @args ) = @_;
    GetOptionsFromArray( \@args, ... );
    $param //= 'World';
    return 'Hello ' . $param;
}
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