The ifdef module would be useful for this. Write your module like this:

package MyClass; {
    # Put class code here inside the braces to
    # constrain the lexical scope of its variables
    ...
}

1;

=begin TEST

package main;
# Test code
my $obj = MyClass->new();
...

=cut
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Then run it like this:

perl -Mifdef=TEST MyClass.pm
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I'm sure others will offer more ideas, too.

Update: Added missing 1; as pointed out below.