It makes sense to move the conditional inside the subroutine as it shouldn't be operating on $str unless it is in the valid format. A small addition that could be made is some additional dwimmery to the subroutine such as modification of the string inplace and working on multiple values e.g 
sub fraction {
  return @_ unless @_ == grep /\d+_\d+/, @_;
  my $args = defined wantarray ? [@_] : \@_;
  $_ = convert_num_here($_)
    for @$args;
  return wantarray ? @$args : $args[-1];
}

[download]
Also, if you're working with fractions davorg's Number::Fraction might be of interest.
HTH

_________
broquaint

In reply to Re: Call sub first or later by broquaint
in thread Call sub first or later by kiat

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