However unless the input corresponds with the first line, the correct information is not printed out in the while loop

shift is destructive on @partners. By doing the shift inside the loop, you're (probably) emptying out the array, ensuring that you'll never get a match.

Since you intend to match against any element of @partners, try replacing your test with

    if ( 1 == grep {$_ eq $username} @partners ) {
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and remove the line that's shifting off $item.