#this is what the config file will look like...
#string to replace, replacement
fred, john
paul, pete
smelly, feet

#!/usr/bin/perl -w
# here is some code (untested)
use strict;
my $config="substit.cfg";
my $regex;

# setup our regexen
open CONF, $config or die "cant open config $!\n";
while (<CONF>) {
    next if /^#/;    # ignore comments
    next if /^\s*$/; # and empties
    chomp;
    my ($replace, $with)=split /,\s*/;
    $regex.="s/".$replace."/".$with."/;";
}
close CONF;

# apply them to the input stream
while (<>) {
    eval $regex;
    print;
}