Please show us the code you have. A possible approach is:

use strict;
my %repl = (
    Jan => 'JOO',
    Fan => 'FOO',
    Janu => 'JOOU',
);

my $search = join "|", map { qr-\b$_\b- } keys %repl;
print "Search regular expression is >>$search<<\n";

my $string = "Jan said: I'm a real Fan of Janus. I call him Janu.";
print "$string\n";

$string =~ s/($search)/$repl{$1}/ge; 

print "$string\n";
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See perlre for further information on /g and /e.

I don't know if it's better or not but I would try to incorporate the capturing parentheses in the generated pattern rather than having to remember to put them in the substitution later.

my $search = do
   {
       local $" = q{|};
       qr{\b(@{ [ keys %repl ] })\b};
   };
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It just seems safer to keep everything together.

Cheers,

JohnGG

Are %repl's keys regexp or text?

If they're regexp, your lookup won't work.

If they're text, they should be converted to regexps before being included in the combined regexp.

my $search = join "|", map { qr-\b\Q$_\E\b- } keys %repl;
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Of course, you're better of using Regexp::List.

use Regexp::List;
my $search = Regexp::List->new->list2re(keys %repl);
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updates. 

Jan have to be replaced with JOO
Feb have to be replaced with FOO