It seems that Win32::API::Struct::Sizeof is returning the wrong size. Setting it to 296 and your code works:

#! perl -slw
use strict;
use Data::Dumper;
use Win32::API;

Win32::API::Struct->typedef( PROCESSENTRY32  => qw{
        LONG dwSize;
        LONG cntUsage;
        LONG th32ProcessID;
        LONG th32DefaultHeapID;
        LONG th32ModuleID;
        LONG cntThreads;
        LONG th32ParentProcessID;
        LONG pcPriClassBase;
        LONG dwFlags;
        CHAR szExeFile[260];
}) ;

Win32::API->Import('kernel32.dll', "HANDLE CreateToolhelp32Snapshot(DW
+ORD dwFlags,DWORD th32ProcessID)");
Win32::API->Import('kernel32.dll', "BOOL Process32First( HANDLE hSnaps
+hot, LPPROCESSENTRY32 lppe )" );
Win32::API->Import('kernel32.dll', "BOOL Process32Next( HANDLE hSnapsh
+ot, LPPROCESSENTRY32 lppe )" );

my $TH32CS_SNAPPROCESS = 0x2;

$^W=0;
my $Snapshot    = undef;
my $ProcessInfo = Win32::API::Struct->new('PROCESSENTRY32') or die $^E
+;

$Snapshot = CreateToolhelp32Snapshot($TH32CS_SNAPPROCESS, 0) or die $^
+E;

$ProcessInfo->{dwSize} = 296; #Win32::API::Struct::sizeof ($ProcessInf
+o);

my $Sucess = Process32First( $Snapshot, $ProcessInfo) or die $^E;
print "First Process : $ProcessInfo->{szExeFile}\n"; #are returning no
+thing
#print Dumper $ProcessInfo;

while( Process32Next( $Snapshot, $ProcessInfo ) ) {
    print $ProcessInfo->{szExeFile};
#    print Dumper $ProcessInfo;
}
__END__
P:\test>446283
First Process : [System Process]

System
SMSS.EXE
CSRSS.EXE
WINLOGON.EXE
SERVICES.EXE
LSASS.EXE
SVCHOST.EXE
SVCHOST.EXE
SVCHOST.EXE
SVCHOST.EXE
vsmon.exe
EXPLORER.EXE
zlclient.exe
TASKMGR.EXE
mdm.exe
CMD.EXE
CMD.EXE
CMD.EXE
CMD.EXE
CMD.EXE
CMD.EXE
opera.exe
perl.exe
[download]

I worked out what was wrong by adding some error checking which produced the error text: "The program issued a command but the command length is incorrect" from the call to Process32First().