Perl doesn't force it upon you, but it's worth the extra effort to put your reusable code in their own unique name space. By doing so, your code is closer to a full fledged Perl module and is self-contained, which helps to avoid name space collisions with the subroutines and global variables of the programs which use it (hopefuly you're avoiding global variables altogether). In fact, I tend to use the same subroutine names in my modules when it makes sense, just to make their use consistent and easier for me to remember.

package My::This;
sub thing { print 'This' }

package My::That;
sub thing { print 'That' }

package main;
&My::This::thing; # prints This
&My::That::thing; # prints That
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It's also just one small step from becoming object oriented...

package My::This;
sub new { bless {} }
sub thing { print 'This' }

package My::That;
sub new { bless {} }
sub thing { print 'That' }

package main;
my $This = My::This->new();
my $That = My::That->new();
$This->thing; # prints This
$That->thing; # prints That
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