You are using $ARGV[0] in your subroutine. That is the first command-line argument to the program, not the parameter to the subroutine. Did you mean that? Arguments are passed into a sub through @_, not @ARGV.

If not, then use one of the following (TMTOWTDI):

$mystruct = shift;   # defaults to @_ in a sub
($mystruct) = @_;    # copies the first element into $mystruct
$mystruct = $_[0];   # Ugly, and possibly misleading
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