Here's how to match and set the pos in one line:

use re 'eval';
my $seq = 'xxxTATAATGyyyTatAtAtzzz';
 my $box = qr{ (?i) TATA [TA] [TA] [TAG] }xms;
 ;;
 my @TATAs;
 my @matches = 
  $seq =~ m{ ($box) (?{ push @TATAs,[$+,pos() - length $+] }) }msxg;

print "matched '$_->[0]' at pos $_->[1]\n" for @TATAs;
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The 'magic' is in the (?{ ... }) construct. This allows you to insert perl into your regexen.