No. \s*M\s* is a regexp. s/\s*M\s*/MALE/si is Perl code (consisting of a substitution operator with a regexp, a replacement string and some options for operands). To run Perl code, you need to use eval EXPR.

A simplistic example:

my $str = 'Hello World!';

# Alias $_ to $str.
foreach ($str) {
   # Assumes each substitute operator is on a different line.
   while (defined(my $code = <DATA>)) {
      eval($code)
         or die("Bad code at input line $.: $@\n");
   }
}

print("$str\n");  # [hello world!]

__DATA__
s/^/[/g
s/$/]/g
s/([A-Z])/lc($1)/eg
[download]

An example allowing the reuse of the substitutions:

my @substitutions;

# Assumes each substitute operator is on a different line.
while (defined(my $code = <DATA>)) {
   push @substitutions, eval("sub { $code }")
      or die("Bad code at input line $.: $@\n");
}

my $str1 = 'Hello World!';
my $str2 = 'Good Day!';

# Alias $_ to the variable.
foreach ($str1, $str2) {
   foreach my $substitution (@substitutions) {
      $substitution->();
   }
}

print("$str1\n");  # [hello world!]
print("$str2\n");  # [good day!]

__DATA__
s/^/[/g
s/$/]/g
s/([A-Z])/lc($1)/eg
[download]