Others have given you the reason for the warning but this snippet causes some concern:

if ( $baseball{yankees}=$baseball{yankees}.$baseball{mets} )
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Assignment inside an if is unusual .. do you mean to use assignment (=) here or do you want to compare strings? If compare, you want the string eq operator not the assignment operator (=) nor the numerical == operator.

if ( $baseball{yankees} eq $baseball{yankees}.$baseball{mets} )  {
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(but that doesn't make much sense with your examples so ... )