$ perl -MGetopt::Long -we 'GetOptions ("a|x" => \$x, "A|y" => \$y);'
Name "main::y" used only once: possible typo at -e line 1.
Name "main::x" used only once: possible typo at -e line 1.
Duplicate specification "A|y" for option "a"
$ perl -MGetopt::Long -we 'GetOptions ("a|x" => \$x, "b|X" => \$y);'
Name "main::y" used only once: possible typo at -e line 1.
Name "main::x" used only once: possible typo at -e line 1.
Duplicate specification "b|X" for option "x"