Beware that you are modifying the scalar passed as $_[0]. You probably want to do:

sub hexdump {
  (my $x= shift) =~ s/\G(.{1,16})/
    (my $s= $1) =~ y|\x20-\x7e|.|c;
    sprintf "%08x  %-50|%s|\n", $-[0],
      "@{[unpack q{(H2)8a0(H2)8},$1]}", $s
  /ges;
  $x
}
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