cd /tmp
mkdir fooa
mkdir foob
touch ./fooa/startlib.pl
touch ./foob/startlib.pl
touch ./foo.pl
export RUNMETH='DEBUG'
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In fooa/startlib.pl

#!/usr/bin/perl -w

our $money = 10000;

1;
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In foob/startlib.pl

#!/usr/bin/perl -w

our $money = 20000;

1;
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In foo.pl

#!/usr/bin/perl -w
BEGIN {
    if ($ENV{'RUNMETH'} eq 'DEBUG'){
        chdir qq~/tmp/working/fooa~;
    } else {
        chdir qq~/tmp/working/foob~;
    }
}

require qq~./startlib.pl~;

print $money;

1;
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Back in my day we had a development, a test, an acceptance, and various production instances. Code and configurations were all dependent on what you were working on. This was a useful way. Perhaps nowadays there is a superior method but, as you can see, this still works.