A nasty solution to the invocation problem is to use eval. If SomeModule comes from the outside world this is a really bad solution!
use strict;
use warnings;
my $modName = 'Getopt::Long';
my $fixedMod = $modName;
$fixedMod =~ s!::!/!g;
$fixedMod .= '.pm';
require $fixedMod;
my $length = 24;
eval "${modName}::GetOptions('length:i' => \\\$length)";
However, there may be a much better solution to your big picture issue. What are you trying to do that seems to need a require?
Optimising for fewest key strokes only makes sense transmitting to Pluto or beyond