The code you have looks about right. This works OK:

#!/usr/bin/perl -w

use strict;

$SIG{__DIE__} = sub {
  print "redirecturl?error='@_'\n";
  exit(1);
};

die "3";
[download]

produces:

redirecturl?error='3 at /tmp/t112 line 10.
'

The problem is that you also get a line number with your error message. You can either eliminate that with a regex, or you can use die "3\n" and then use chomp to eliminate the newline. The @_ variable is readonly, so you'll have to make a copy:

#!/usr/bin/perl -w

use strict;

$SIG{__DIE__} = sub {
  my $why = shift;
  $why =~ s/( at.*)?\n$//;
  print "redirecturl?error='$why'\n";
  
  exit(1);
};

die "3";
[download]

produces

redirecturl?error='3'

whether you use die "3" or die "3\n".