For imsx, you can use (?switches:pattern). For g, you'll have to use an if.

my $source = "abc";
my $find = "a";
my $replace = "x";
my %opts = (
   i => 1,
   m => 0,
   s => 0,
   x => 0,
   g => 1,
);

my $opts = join '', map { $opts{$_}?$_:'' } qw( i m s x );
my $re = "(?$opts:$find)";

if ($opts{g}) {
   $source =~ s/$re/$replace/g;
} else {
   $source =~ s/$re/$replace/;
}

print $source;
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Update: Fixed where %opts was referenced as %opt.