Not exactly sure whether you meant $TEST or TEST, but maybe you want 

if ( defined &TEST::test_sub ) {           # package TEST

if ( defined &{"${TEST}::test_sub"} ) {    # package in $TEST

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(The One :) difference between defined &{"${TEST}::test_sub"} and $TEST->can("test_sub") is that the latter would consider inheritance, while the former would not — so which one to use depends on what exactly you want...)

In reply to Re: How to determine if a package method is defined by almut
in thread How to determine if a package method is defined by Anonymous Monk

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