Note that this issue (already explained by others) isn't limited to $1 and frieds. It can happen to other variables as well, including lexical scoped variables:

  use 5.010;

  use strict;
  use warnings;
  my($foo, $bar) = (1, 2);

  sub baz {
      say shift;
      $bar = 3;
      say shift;
  }

  baz $foo, $bar;

  __END__
  1
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