$a =~ $b

##</code><code>##

$a =~ m/$b/;

##</code><code>##

$p = '4+5*6';   $str =~ m/$p/;   # Doesn't perform arithmetic.
$p = 'm/a/';    $str =~ m/$p/;   # Doesn't search for `a`.
$p = 's/a/b/';  $str =~ m/$p/;   # Doesn't perform a substitution.

##</code><code>##

$p = '4+5*6';   $str =~ $p;      # Doesn't perform arithmetic.
$p = 'm/a/';    $str =~ $p;      # Doesn't search for `a`.
$p = 's/a/b/';  $str =~ $p;      # Doesn't perform a substitution.

##</code><code>##

$p = '4+5*6';   $re = qr/$p/;  $str =~ m/$re/;   # Still doesn't.
$p = 'm/a/';    $re = qr/$p/;  $str =~ m/$re/;   # Still doesn't.
$p = 's/a/b/';  $re = qr/$p/;  $str =~ m/$re/;   # Still doesn't.

##</code><code>##

$p = '4+5*6';   $re = qr/$p/;  $str =~ $re;      # Still doesn't.
$p = 'm/a/';    $re = qr/$p/;  $str =~ $re;      # Still doesn't.
$p = 's/a/b/';  $re = qr/$p/;  $str =~ $re;      # Still doesn't.

##</code><code>##

my $pat = 'a';

my $re = qr/$pat/;

$str =~ m/$re/    # Performs a match.
$str =~ /$re/     # Same
$str =~ $re       # Same 

$str =~ m/$pat/   # No need to compile in advance.
$str =~ /$pat/    # Same
$str =~ $pat      # Same 

##</code><code>##

my $pat  = 'a';
my $repl = 'b';

my $re = qr/$pat/;

$str =~ s/$re/$repl/    # Performs a substitution.

$str =~ s/$pat/$repl/   # No need to compile in advance.