foreach (qq/frog toad/, qq/man boy/, qq/woman girl/) {
    my ( $pattern, $substitution) = split /\s/, $_;
    $foo =~ s/\Q$pattern\E/$substitution/g;
}

##</code><code>##

my %testhash = ( qw/frog toad man boy woman girl/ );
foreach (keys %testhash) {
    $foo =~ /\Q$_\E/$testhash{$_}/;
}