I found that writing tests along with the code helps me catch error conditions early. I use Test::More. Sometimes, the code I write is not intended to run in my development/test environment. In these cases I modify package objects, or data structures within the tests to get the desired scenario. Then I run a test against it.

Test::More example:

foo.pl contains a function named foo_plus_one like this -

     sub foo_plus_one($) { return $_[0]+1 }
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You can 'require' your script into the test -

     BEGIN { require_ok("('foo.pl')") };
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and call the functions -

     my $foo = 4;
     is( foo_plus_one($foo), '5', 'method foo_plus_one()' );
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pass output -
1..1
ok 1 - method foo_plus_one()

OR fail -
1..1
not ok 1 - method foo_plus_one()
# Failed test (foo.t at line 6)
# got: '1003'
# expected: '5'


Testing is a great way to build a program! Ye-haw!