From Getopt::Long:

 Using a colon ":" instead of the equals sign indicates that the
option value is optional. In this case, if no suitable value is 
supplied, string valued options get an empty string '' assigned, while
+ 
numeric options are set to 0.
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Update: Something like

use strict;
use warnings;
use Getopt::Long;
my $foo;

GetOptions ('foo:s', \$foo);

print "foo: $foo\n" if defined($foo);

print "Unprocessed by Getopt::Long\n" if $ARGV[0];
foreach (@ARGV) {
  print "$_\n";
}
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Note that you should check for definedness rather than truth in $foo as the default values for optional parameters (0 and '') are false values.