#!/usr/bin/perl

use feature 'switch';
use strict;

require "lib.pl";

my $global_variable1='a';
my $global_variable2='b';
my $global_variable3='c';

given( subroutine1() ){
     when (0) { subroutine2($global_variable1) }
     when (1) { subroutine2($global_variable2) }
     when (2) { subroutine2($global_variable3) }
}

#!/usr/bin/perl
# lib.pl

use strict;

sub subroutine1 { return rand(3); }
sub subroutine2 { print shift; }
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or, even better...

#!/usr/bin/perl

use strict;

require MyLib;

my @global_variable = qw( a b c );
subroutine2( $global_variable[subroutine1()] );

#!/usr/bin/perl
# MyLib.pm
use strict;
sub subroutine1 { return rand(3); }
sub subroutine2 { print shift; }
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