Something like:

s/\b(find)\b/$1 eq uc($1) ? "REPL" : $1 eq ucfirst(lc($1)) ? "Repl" : 
+"repl"/gie
[download]

or more flexibly:

my %repl = (
    find => 'repl',
    FIND => 'REPL',
    Find => 'Repl',
);
s/\b(find)\b/$repl{$1}||$repl{lc $1}/gie
[download]