use Devel::Peek qw( Dump );

sub f {
   Dump( $_[0] );
   # ...
}

my $x = "abc";
Dump( $x );
f( $x );            # Only a "C pointer" copied onto the stack.

##</code><code>##

SV = PV(0x55df7c8f0ee0) at 0x55df7c91f1e0    <-- One scalar
  REFCNT = 1
  FLAGS = (POK,IsCOW,pPOK)
  PV = 0x55df7c94f190 "abc"\0
  CUR = 3
  LEN = 16
  COW_REFCNT = 1
SV = PV(0x55df7c8f0ee0) at 0x55df7c91f1e0    <-- Same scalar
  REFCNT = 1
  FLAGS = (POK,IsCOW,pPOK)
  PV = 0x55df7c94f190 "abc"\0
  CUR = 3
  LEN = 16
  COW_REFCNT = 1

##</code><code>##

use Devel::Peek qw( Dump );

sub f {
   my $x = shift;   # Scalar is copied.
   Dump( $x );
   # ...
}

my $x = "abc";
Dump( $x );
f( $x );            # Only a "C pointer" copied onto the stack.

##</code><code>##

SV = PV(0x5602d6441ee0) at 0x5602d6470148    <-- One scalar
  REFCNT = 1
  FLAGS = (POK,IsCOW,pPOK)
  PV = 0x5602d6478030 "abc"\0                <-- String buffer
  CUR = 3
  LEN = 16
  COW_REFCNT = 1
SV = PV(0x5602d6441f00) at 0x5602d6470100    <-- Different scalar
  REFCNT = 1
  FLAGS = (POK,IsCOW,pPOK)
  PV = 0x5602d6478030 "abc"\0                <-- String buffer
  CUR = 3
  LEN = 16
  COW_REFCNT = 2