• my $ns = GetNamespace("MAPI");
  should probably be
  my $ns = $Outlook->GetNamespace("MAPI");

• Yes, return; is indeed the equivalent of Exit Sub.

• Your indentation is inconsistent, making your code hard to read.

• $i serves no purpose. Same as in the VB code.

• $ol is never used either.

• $Atmt is never assigned a value. Change

  my $Item;
  my $Atmt;
  ...
  foreach (in $Inbox->Items) {
     foreach (in $Atmt->Item->Attachments) {
  [download]

  to

  foreach my $Item (in $Inbox->Items) {
     foreach my $Atmt (in $Item->Attachments) {
  [download]

• The following (and the VB equivalent) is useless since the variables get freed when they go out of scope:

  undef $Atmt;
  undef $Item;
  undef $ns;
  [download]

• The return at the end of the function is useless.

So we get:

#! perl

use strict;
use warnings;

use File::Spec ();
use Win32::OLE qw( in ); 
use Win32::OLE::Const 'Microsoft Outlook';

my $SAVE_FOLDER = 'G:\Input';

my $Outlook = Win32::OLE->new('Outlook.Application', 'Quit');

Detach_files();

sub Detach_files {
   my $ns = $Outlook->GetNamespace('MAPI');
   my $Inbox = $ns->GetDefaultFolder(olFolderInbox);

   return if $Inbox->Items->{Count} == 0;
    
   foreach my $Item (in $Inbox->Items) {
      # Check each message for attachments.
      foreach my $Atmt (in $Item->Attachments) {
         # Save any "Flat_file_" attachments found.
         my $FileName = $Atmt->FileName;
         if ($FileName =~ /^Flat_file_/) {
            $FileName = File::Spec->catdir($SAVE_FOLDER, $FileName);
            $Atmt->SaveAsFile($FileName);
         }
      }
   }
}
[download]

Untested.

I believe you need to say

$ns = $Outlook->GetNamespace('MAPI');
[download]