I think that if the only time you call fraction() is if you have performed the test, then putting the test in the fraction() sub is just fine.

Now, personally, I would do the following:

$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;
}
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I like my functions to be atomic, but that's just me. Either way is maintainable.