You can use diagnostics module to understand many warnings and errors...

Using this code:

use warnings;
use strict;

sub a{ 1 };
sub a{ 2 };
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Diagnostics says:

$ perl -Mdiagnostics x
Subroutine a redefined at x line 6 (#1)
    (W redefine) You redefined a subroutine.  To suppress this warning
+, say

        {
        no warnings 'redefine';
        eval "sub name { ... }";
        }
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.............