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";