Some hints:

• Check the "perlvar" man page to read about the "INPUT_RECORD_SEPARATOR" variable, "$/". You can set it to read each block of lines (up to the next blank line) instead of one line at a time, so each element of your "@all" array contains a full config record.
• Look up the "split" function (perldoc -f split). After reading each block of lines into each array element, separate the lines ( split /\n/), then split each non-initial line into "attribute" and "value" ( split /=/).
• You can load your array of hashes while reading the file, so you don't end up with two copies of the data in memory and you don't need to do repeated loops over the same set of data.

At the risk of short-cutting your exploration of the essential man pages, it could end up looking like this:

my @AoH_attribs;

open( FILE, "<$file" );
{
   local $/ = '';  # empty string is "magic" for "paragraph-mode"

   while (<FILE>)   # read a group of lines into $_
   {
      my %hash = ();
      my $itemname = ( /^\[(.*?)\]/ ) ?  $1 : 'NO_KEY';
      next if ( !/=/ or $hashkey eq 'NO_KEY' );

      $hash{item} = $itemname;

      for my $line ( split /\n/ ) {
         next unless ( $line =~ /=/ );

         my ( $attrib, $value ) = split /=/, $line;
         $hash{$attrib} = $value;
      }
      push @AoH_attribs, { %hash };  # note use of curly braces
   }
}
close FILE;

# now, @AoH_attribs contains a list of hash refs, one for each input b
+lock;
# you can access each hash like this:

for my $itemref ( @AoH_attribs ) {
   my %itemhash = %$itemref;
   print "$_ = $itemhash{$_}\n" for ( sort keys %itemhash );
   print "\n";
}
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There are other ways as well.

(update: fixed syntax error in code)