Do exactly what you've just said. If no parameter specified to your program, just request the user for input. And unless there's no input either, simply quit the program then ;)

my $foo_arg = $ARGV[0]; # or just shift();
unless ($foo_arg) {
  print "Foo arg is missing! Please enter:\n";
  $foo_arg = <STDIN>;
  chomp($foo_arg);
  die "Alright, i'm just quitting this one!" unless $foo_arg;
}
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_____________________

$"=q;grep;;$,=q"grep";for(`find . -name ".saves*~"`){s;$/;;;/(.*-(\d+)
+-.*)$/;
$_=["ps -e -o pid | "," $2 | "," -v "," "];`@$_`?{print"+ $1"}:{print"
+- $1"}&&`rm $1`;
print$\;}
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