That's how magic variables works. Every time you can read a variable with get magic, a getter function is called to populate it first. Every time you write to a variable with set magic, a setter function is called to process the new value afterwards. 

use v5.40;

use Variable::Magic qw( cast wizard );

my $wiz = wizard(
   get  => sub {
      say sprintf 'getter called for %X', refaddr( $_[0] );
      ${ $_[0] } = int( rand( 100 ) );
   },
);

my $var;
say sprintf '`$var` is %X', refaddr( \$var );

cast $var, $wiz;

for ( 1 .. 4 ) {
   say "Loop: $_";
   say "`\$var` has value $var";
}

[download]
`$var` is 5F5668128E20
Loop: 1
getter called for 5F5668128E20
`$var` has value 22
Loop: 2
getter called for 5F5668128E20
`$var` has value 62
Loop: 3
getter called for 5F5668128E20
`$var` has value 44
Loop: 4
getter called for 5F5668128E20
`$var` has value 70

[download]

The alternative to magic would be to preemptively copy substrings of the matched string into $`, $&, $' and $n, $+{name} and $-{name}.

In reply to Re^3: Why is "any" slow in this case? by ikegami
in thread Why is "any" slow in this case? by Anonymous Monk

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