How you're trying to do this will not work, with or without a BEGIN block. use is evaluated at compile time, and both use libs in that code fragment will need to be compiled, and thus will be executed. Other ways:

use lib (index($ENV{SCRIPT_NAME},'/dev/') == -1) ?
    "/blah/perllib/" : "/blah/perldev/";
# -- or --
BEGIN {
  require lib;
  if ( index( $ENV{SCRIPT_NAME}, '/dev/' ) == -1 ) {
      lib->import("/blah/perllib/");
    } else {
      lib->import("/blah/perldev/");
    }
  }
}
# -- or --
BEGIN {
  if (index( $ENV{SCRIPT_NAME}, '/dev/' ) == -1 ) {
      unshift @INC, "/blah/perllib/";
    } else {
      unshift @INC, "/blah/perldev/";
    }
  }
}
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