Although you already have your answer for the given code I should point out the inefficiency of repeatedly running regexes in that fashion:

use strict;
use warnings;
use Benchmark qw|timethese cmpthese|;

my $a = "aRep";
my $b = "bRep";
my $c = "cRep";

my @Search = ("?a?", "?b?", "?c?");
my @Search1 = (qr|\?a\?|,qr|\?b\?|,qr|\?c\?|);
my @Replace = ($a,$b,$c);

my $i=0;
my $string = '?a? ?b? ?c?';
print "old string: $string\n";

print qq~ one --- \n~;
&std_replace;
print qq|String is now $string\n|;

print qq~ two --- \n~;
&improved_replace;
print qq|String is now $string\n|;

print qq~ Comparison --- \n~;

timethese (100_000, {'STD_REPLACE'=>\&std_replace,
                    'IMP_REPLACE'=>\&improved_replace});

sub std_replace {
     $string = '?a? ?b? ?c?';
     for (my $i = 0; $i < $#Search + 1; $i++) {
         $string =~ s/\Q$Search[$i]/$Replace[$i]/g;
     }
}

sub improved_replace {
     $string = '?a? ?b? ?c?';
     for (;$i<$#Search1+1;$i++) {
    $string =~ s/$Search1[$i]/"$Replace[$i]"/g;
     }
}
1;
[download]

old string: ?a? ?b? ?c?
 one --- 
String is now aRep bRep cRep
 two --- 
String is now "aRep" "bRep" "cRep"
 Comparison --- 
Benchmark: timing 100000 iterations of IMP_REPLACE, STD_REPLACE...
IMP_REPLACE:  0 wallclock secs ( 0.10 usr +  0.00 sys =  0.10 CPU) @ 1
+000000.00/s (n=100000)
            (warning: too few iterations for a reliable count)
STD_REPLACE:  6 wallclock secs ( 4.99 usr +  0.00 sys =  4.99 CPU) @ 2
+0040.08/s (n=100000)
[download]

But I suppose this only matters when the number of comparisons increases significantly.