Question: do any of your search strings contain regex control characters (., (, [ ...)? If so, you need to modify your regular expression line to

$replacecount += ( $line =~ s/\Q$key\E/$table_ref->{$key}/g );

to avoid unintended consequences. It's probably a good idea any, just in case. See String interpolation to see a bit more about how interpolation works in Perl.

Regarding the main part of your question, I read your issue as "I want all substitutions to only occur if they would have occurred on the original file". Might I suggest, then caching the original line and testing if you would have a match before substituting? You could put an error statement in there as well to make sure the number of substitutions matched the number of expected substitutions. Something like: 

# I have already read the 2-column table file into %table.
# I've also already opened INFILE and OUTFILE

my @keys_ordered = sort { length $table_ref->{$b} <=>
    length $table_ref->{$a} } keys %{$table_ref};
# Sorted in reverse order of length, so that longer strings
# in the data file are found before shorter strings
# Done here to increase speed, vs inside the while loop

my $replacecount = 0;
while ( my $old_line = <INFILE> ) {
    my $new_line = $old_line;
    for my $key ( @keys_ordered ) {
        my $old_count = my $new_count = 0;
        if ($old_count = ($old_line =~ /\Q$key\E/g)) {
            $new_count = ( $new_line =~ s/\Q$key\E/$table_ref->{$key}/
+g );
        }
        if ($new_count != $old_count) {
            die "Match count failure ($old_count/$new_count) on $line"
        }
        $replacecount += $new_count;
    }
    print OUTFILE $new_line;
}    
print "Made $replacecount replacements.\n";

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In reply to Re: Algorithm to search and replace on data file, based on a table file? by kennethk
in thread Algorithm to search and replace on data file, based on a table file? by koknat

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