morgon:   Further to Corion's post:

[Corion]: The match runs to completion before the map even starts.

At the time your map runs, $1 and $2 are what they are at the end of the global match.

Here's an instrumented version of the OPed code to better (I hope) show what's happening:

c:\@Work\Perl\monks>perl -wMstrict -le
"use Data::Dumper;
 ;;
 my $s = \"1 A\n2 B\n33 foo\n\";
 ;;
 my %h =
   map { print qq{in map: \$_ '$_'  \$1 '$1'  \$2 '$2'};  $1 => $2; }
   $s =~ /(\d+)\s+(\S+)/mg
   ;
 print Dumper(\%h);
"
in map: $_ '1'  $1 '33'  $2 'foo'
in map: $_ 'A'  $1 '33'  $2 'foo'
in map: $_ '2'  $1 '33'  $2 'foo'
in map: $_ 'B'  $1 '33'  $2 'foo'
in map: $_ '33'  $1 '33'  $2 'foo'
in map: $_ 'foo'  $1 '33'  $2 'foo'
$VAR1 = {
          '33' => 'foo'
        };
[download]

(Note that this example doesn't take into account the revised data format specification suggested here. :)