my %subs = (
    '\?a\?' => 'aRep',
    '\?b\?' => 'bRep',
    '\?c\?' => 'cRep',
);

for my $match (keys %subs) {
    $string =~ s/$key/$subs{Skey}/g;
}

##</code><code>##

my @subs = (
    ['\?a\?' => 'aRep'],
    ['\?b\?' => 'bRep'],
    ['\?c\?' => 'cRep'],
);

for my $match (@subs) {
    $string =~ s/$match->[0]/$match->[1]/g;
}