in reply to searching for elements

Read the error message once more: the argument of exist cannot be an array. @{$pop{$element}{interactions}} is an array.

If you want to check the whole array, use the any function fom List::MoreUtils.

CountZero

A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James

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In Section Seekers of Perl Wisdom