Define your library as a module, using the use statement instead of a require statement.

Library:

package Subs;
our $Foo = 3;
sub foo { $Foo++; }
1;
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Program:

use Subs;
Subs::foo();
print $Subs::Foo, "\n";
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If you want to then export the symbols &Subs::foo and $Subs::Foo to the main program, you add a little stuff to your library. The Exporter module helps you export certain symbols into the "parent" or calling scope.

Library:

package Subs;
BEGIN {
  use Exporter;
  our @ISA = qw(Exporter);
  our @EXPORT = qw($Foo &foo);
}
our $Foo = 3;
sub foo { $Foo++ }
1;
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Program:

use Subs;
foo();
print $Foo, "\n";
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For a lot more detail on this process, and other features, check out perlmod (or type perldoc perlmod at your console).