s/(key1|key2|key3)/$hash{$1)/g;

##</code><code>##

my $re = join '|', keys %dict;
$re = qr|($re)|;
s/$re/$hash{$1}/g;

##</code><code>##

use Regexp::Assemble;
my $ra = Regexp::Assemble->new;
$ra->add($_) for keys %dict;

##</code><code>##

s/(\w+)/$dict{$1}||$1/ge;

##</code><code>##

s/(\w+)(?(?{$dict{$1}})|(?!))/$dict{$1}/g;