$a = "aaabbbaaa";
$b = "s/b/c/g";
eval '$a =~ ' . $b;

##</code><code>##

local $_ = "aaabbbaaa";
$b = "s/b/c/g";
eval $b;

##</code><code>##

# Inputs
# ======

$string = "aaabbbaaa";
$regexp = "b";
$subst  = "c";
$global = 1;

# (?i...) can be used in $regexp for /.../i
# (?m...) can be used in $regexp for /.../m
# (?s...) can be used in $regexp for /.../s
# (?x...) can be used in $regexp for /.../x
# /g cannot be stored in $regexp, thus the need for $global.
# /e cannot be stored in $regexp, but I'm not supporting it.

# Guts
# ====

# Check user input.
eval {
   # Let's explicitely be on the safe side.
   no re 'eval'; 
   $regexp = qr/$regexp/;
};

# Handle bad user input.
die("Bad regexp supplied: $@\n") if $@;

# Do the work.
if ($global) {
   $string =~ s/$regexp/$subst/g;
} else {
   $string =~ s/$regexp/$subst/;
}