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my $texttosub = "log10(blackcat)";
#the value of "blackcat" can be found in a hash called "%cats"
...
  ..do something..
}
sub log10{....}

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