It's a precedence issue, the && is binding stronger than you think. Use extra parens, or use and, which has lower precedence. See perlop for the precedence table. Here's an explanation-by-code:

$ perl -MO=Deparse,-p -e 'substr ( $_,2,8 ) = "        " && substr ( $
+_,15,5 ) = "     " if /^AB/'
(/^AB/ and substr($_, 2, 8) = (substr($_, 15, 5) = '     '));
$ perl -MO=Deparse,-p -e 'substr ( $_,2,8 ) = "        " and substr ( 
+$_,15,5 ) = "     " if /^AB/'
(/^AB/ and ((substr($_, 2, 8) = '        ') and substr($_, 15, 5) = ' 
+    '));
$ echo "AB2014052012345HELLOWORLD" | perl -pe 'substr ( $_,2,8 ) = "  
+      " && substr ( $_,15,5 ) = "     " if /^AB/'
AB     12345     WORLD
$ echo "AB2014052012345HELLOWORLD" | perl -pe 'substr ( $_,2,8 ) = "  
+      " and substr ( $_,15,5 ) = "     " if /^AB/'
AB        12345     WORLD
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