Construct a regex from the hash keys, and do a match with them. 
$pat =
  join '|',
  sort { length($b) <=> length($a) }
  keys %prefix;

if ($number =~ /^($pat)/) {
  print "Prefix: $1\n";
}

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In reply to Re: Fast lookup for prefixes in hash by japhy
in thread Fast lookup for prefixes in hash by Marcello

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