You could do

use lib 'modules_src';
use MyPackage;
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or

BEGIN {
    require 'modules_src/MyPackage.pm';
    # (call import() here, if required...)
}
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(assuming your current working directory is set such that it contains modules_src; otherwise specify the absolute path)

Though, personally, I prefer to stick to the convention, i.e. if your module is named Sth::MyPackage, then put it in a subdirectory named accordingly: modules_src/Sth/MyPackage.pm. Then, you can write

use lib 'modules_src';
use Sth::MyPackage;
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and have your use statement properly indicate the module's namespace...

The overall code directory structure look simply like this:

From the docs (perldoc -f require):

In other words, if you try this:

require Foo::Bar; # a splendid bareword

The require function will actually look for the "Foo/Bar.pm" file in the directories specified in the @INC array.

And use is "an enhanced version of require", so to speak. See perlfaq8 for differences between do, require and use.

You can add the parent of Sth (say, ~/MyPerl/modules_src) with

use lib '/home/neversaint/MyPerl/modules_src'; # no tilde expansion
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or including that path in your PERL5LIB environment variable.

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

The other suggestions are correct, you want to:

use lib '/path/to/modules_src';

But this isn't much fun maintanence wise, because when you move around your application directory you have to update this line of code. The FindBin module, which has come with perl for a while now, takes care of this problem:

use FindBin;
use lib "$FindBin::Bin/../modules_src";
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This way you can move around the application directory at will and @INC will get set properly without modifying the code.

EDIT:

Based on your application directory's structure, you would want to use:

use FindBin;
use lib "$FindBin::Bin/modules_src";
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Because the program you are ruuning is in the root of your application directory

unfortunately FindBin is not very reliable, especially in a persistent web environment (mod_perl). this has been discussed before:

• FindBin is broken (RE: How do I get the full path to the script executing?)
• $Bin undef with FindBin in web environment

the correct method, learned from the monks, that i currently use without problems in shell/web programs:

use vars qw( $RealBin );

BEGIN {
    use File::Spec::Functions qw( rel2abs splitpath catpath);
    $RealBin = catpath ((splitpath (rel2abs ($0)))[0,1]);
}

use lib "$RealBin/modules_src/";  # relative path from your Perl progr
+am
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:)))))

That works, more or less, but gives me a warning, on Windows/perl5.6.1: Use of uninitialized value in concatenation (.) or string at [...]/lib/File/Spec/Win32.pm line 240.

The reason is the lack of the third parameter to catpath.

What's more, it still has a trailing backslash (probably a slash on Unix), which is not my idea of ideal. It might work, or it might fail on some platforms.

The following code no longer produces the warning, but I still get the trailing backslash:

$RealBin = catpath ((splitpath (rel2abs ($0)))[0,1], '');
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File::Spec is dumb.