$regexp = "a";
$subst  = "b";
$global = 1;

if ($global) {
   $string =~ s/$regexp/$subst/g;
} else {
   $string =~ s/$regexp/$subst/;
}

# (?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.