This line:

my @FileErrors = grep { exists $ErrorStrings{$_} } @Output;
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is looking for an exact match of the output line in the hash (i.e. with an hash key), which is probably never going to occur (at the very least, the output line will have a trailing end of line character, but probably also some other words, such as 'XORDER' in your example). You need to use a loop with a regex.