CTRL-Z (end of file) does not break you out of your infinite loop ... so what construct would? Maybe checking if $var has a value? Maybe moving that check into your while so you do not have an infinite loop:

  while( $val = <> )
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There are many ways. Your problem has nothing to do with arrays but with how to write effective control structures. *And* there are more idiomatic ways of figuring out if a value is in an array. Iterating through the array is going to be costly at some point -- you might be better off using a hash to track what's been placed in the array (and ... this really looks like a homework problem).