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$matching_key=grep { $_ =~ /$regex/  } keys %tax2loc;

@all_matching_keys = grep { $_ =~ /$regex/  } keys %tax2loc;
$matching_key = $all_matching_keys[0];

($matching_key) = grep { $_ =~ /$regex/  } keys %tax2loc;

$matching_key = first { $_ =~ /$regex/  } keys %tax2loc;

$matching_key = first { /$regex/ } keys %tax2loc;