Your code as posted (almost) works. After modifying it to

#!/usr/bin/perl -w

use strict;

my $out = '<?xml version="1.0" encoding="utf-8"?>';
my @tmp = split /</, $out;
foreach(@tmp) {
  if (substr($_, 0, 1) eq "?") {
    print "conditionA matched - looping!\n";
    next;
  } elsif (substr($_, 0, 1) eq "!") {
    print "conditionB matched\n";
  }
  print "End of loop\n";
}
[download]

I get the output

End of loop
conditionA matched - looping!
[download]

as expected. (Note that the split is returning a list of two scalars, not just one: the 0 characters before the < and the n characters after it.)

Side note: That feels a lot like C. Changing your tests to if (/^\?/) and if (/^!/) would feel perlier. Should work either way, though.