Simply put the characters into quotes, as Perl does not want barewords.

my $enum_proc = new Win32::API( 'psapi.dll', 'EnumProcesses', [qw[P N 
+P]], qw[I] ) 
  or die "Cannot Enum Processes: $!\n";

# or

my $enum_proc = new Win32::API( 'psapi.dll', 'EnumProcesses', ["P", "N
+", "P"], "I" ) 
  or die "Cannot Enum Processes: $!\n";

# or, slightly confusing

my $enum_proc = new Win32::API( 'psapi.dll', 'EnumProcesses', [ P => N
+ => P => ], "I" ) 
  or die "Cannot Enum Processes: $!\n";

# or, slightly perverse
use constant P => "P";
use constant N => "N";
use constant I => "I";

my $enum_proc = new Win32::API( 'psapi.dll', 'EnumProcesses', [ P, N, 
+P ], I ) 
  or die "Cannot Enum Processes: $!\n";
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Of course, the last proposal is seen to be as a joke, because real fun will start as soon as you lazily name a file handle P and try to read/write with it.

perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ;    # The  
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new   #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' #  web
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