improve robustness regarding undef values
observed output:
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Use of uninitialized value $head_of_actual_ie in pattern match (m//) at /usr/local/bin/rosettaDeepFixity/deep_fixitycheck.pl line 428.
Use of uninitialized value $head_of_actual_ie in concatenation (.) or string at /usr/local/bin/rosettaDeepFixity/deep_fixitycheck.pl line 430.
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