Building upon james2vegas's solution and explanation, and satisfying all of your stated requirements:

• If -foo is not on the command line, leave $foo undefined.
• If -foo is on the command line without an argument, set $foo to a default value of 1, without the error message.
• If -foo is on the command line with an argument, set $foo to the argument.

use strict;
use warnings;
use Getopt::Long;

my $foo;
GetOptions ('foo:s', \$foo);
if (defined $foo) {
   $foo = 1 if $foo eq '';
   print "foo: $foo\n";
}

print "Unprocessed by Getopt::Long\n" if $ARGV[0];
foreach (@ARGV) {
  print "arg=$_\n";
}
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