$str = convert_to_fraction_if_needed( $str );

sub convert_to_fraction_if_needed
{
    my $str = shift;

    return $str unless $str =~ /\d+_\d+/;
    return convert_to_fraction( $str );
}

sub convert_to_fraction
{
    my $str = shift;
    # Do stuff here
    return $converted_str;
}