$str = fraction($str) if ($str =~ /\d+_\d+/);

##</code><code>##

$str = fraction($str);
...
sub fraction {
   my $str = shift;
   if ($str =~ /\d+_\d+/) {
      # convert $str to fraction using mimetex syntax
   }
   return $str;
}

##</code><code>##

if ($str =~ /\d+_\d+/) {
   $str = fraction($str)
} elsif ( condition 2 ) {
   action 2
} else {
   default
}

##</code><code>##

$str = fraction($str)       if ($str =~ /\d+_\d+/)

##</code><code>##

convert_ratio_to_decimal( \$str );

##</code><code>##

sub convert_ratio_to_decimal (\$) { ..... };

convert_ratio_to_decimal( $str );

##</code><code>##

$obj->convert_ration_to_decimal();

##</code><code>##

$error->logBadDateError( $str ) if (ErrorTracking::invalidDate( $str ))

##</code><code>##

$varName = qr(regex goes here);
if ( $str =~ /$UnderscoredNumber/ ) ...       # poor
if ( $str =~ /$SimpleRatio/ ) ...             # good
if ( $str =~ /$Ratio::simpleFraction/ ) ...   # better