It depends. If you want to use YourModule.pm as library of functions, try this - it supposes that YourModule.pm is on the place, which perl seeks for modules:

package MainModule;
use YourModule ();
my $ret = YourModule::yourFunction();
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but, if you want to have an object, which inherits from YourModule.pm, you can try something as:

package MyObject;
use base 'YourModule';

sub new {
...}

my $instance = MyObject->new();
my $ret = $instance->yourFunction();
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and all methods from YourModule will be accessible through $instance.