Your problem is that you're doing an exact match (with eq) against the alphabetic characters but not the terminal newline. For example, if I type in jaccard, then $metric will be the string "jaccard\n". You should probably chomp the input prior to processing it, e.g.

my $metric = <>;
chomp $metric;
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Example of your code as posted:

Which Metric jaccard or Bleu:jaccard
Which source title or snippet or url:title
bleu not called
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Example after chomping $metric and $source:

Which Metric jaccard or Bleu:jaccard
Which source title or snippet or url:title
in title
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P.S. I also notice you prompt for "Bleu" but test for "bleu": these strings are not equal.