This does what you want, but your users have to know about Ctrl-D:

my $argument = $ARGV[0] || (print( "Parameter? "), (<>)[0]);
chomp $argument;
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Note that the comma after print() is the sequence operator here.

Update: chomped at ++tadman's suggestion. Also, this removes the Ctrl-D problem:

my $argument = $ARGV[0] || (print( "Parameter? "), <>);
chomp $argument;
[download]

After Compline,
Zaxo