my $texttosub = "log10(blackcat)";
#the value of "blackcat" can be found in a hash called "%cats"
while ( $texttosub =~ s|([a-zA-Z][A-Za-z_0-9]+)|$cats{$1}|i ){          
  print ("\n", " The value of cat = ", eval ($texttosub) );
  ..do something..
}
sub log10{....}

##</code><code>##

 while ( $texttosub =~ s/(?!log10)|([a-zA-Z][A-Za-z_0-9]+)/$cats{$1}/i ){
   ..do something..
  }