sub mySub
{
     if( $_ =~ /\.html)
        {
          my $name = $File::Find::name;
        open ( F, $name ) || warn "Can\'t open File $name: $!\n";
            
        while($line = <F>) 
        {
            for $hit ($line =~ /matchdata/gi) 
            {
           
                push @files, $name;

                }
         }
         close F;
    } 
}    
        

find( \&mySub, $dir  );



foreach  (@files)
{
    open(LDATA, "$_") || warn "File does not open: $!\n";
    @data = (<LDATA>);
    close(LDATA);
    open(LDATA, ">$_") || warn "File Write problem $_: $!\n";
    
    foreach (@data)
    {
        s/OLD/NEW/gi;
        print LDATA $_;
    }
    close(LDATA);
}
[download]

I can't speak directly to efficiency as it's hardly my specialty, but I would suggest a change to this chunk:

    while($line = <F>) 
        {
            for $hit ($line =~ /matchdata/gi) 
            {
           
                push @files, $name;

                }
         }
         close F;
[download]

Logically it seems like you want to check to see if the file you're currently examining has data that needs to be updated (you're saving the filename in @files for later processing) -- but you're pushing the filename onto your list once for each match in the file! Once you find a match in a particular file you should (based on my understanding of your goal) (1) push the filename onto your to-be-processed stack, and then (2) move on to the next file. So:

    while($line = <F>) 
        {
            if ($line =~ /matchdata/i) 
            {
           
                push @files, $name;
                last;

                }
         }
         close F;
[download]

Note that I dropped the global flag for the match operator, too -- you just want to know if there's something there, anywhere, to be fixed later, and then move on.

Naturally, if I misunderstand your goals this could be way off base :)

Couple of things:
1. if /matchdata/ has any capturing () than $name is pushed into @files for each ().
2. When you do @data = (<LDATA>) you read the entire file into memory. A better way to do this would be:

foreach  (@files)
{
    open(LDATA, "$_") || warn "File does not open: $!\n";
    open(TMP, ">$_.tmp") || warn "File Write problem $_.tmp: $!\n";
    
    while (<LDATA>)
    {
        s/OLD/NEW/gi;
        print TMP $_;
    }
    close(LDATA);
    close(TMP);
    rename("$_.tmp", "$_") or warn "Could not rename '$_.tmp' to '$_':
+ $!\n";
}
[download]

Another side advantage of this is that if, for some reason, your script dies half way through then you still have our old file rather than a half written new file. hth.

$will->code for @food or $$;