In case no header line starts with whitespace followed by "<", but the first non-header line does, a simple solution would be as follows. (Maybe I misunderstood your notation, and the lines do not really start that way, but then either the must be identifiable using another regex or you must resort to counting lines.)

my $in_header++;
while (<IN>) {
  if ($in_header && !/^\s+</) {
    push(@hdr, $_);
  }
  else {
    $in_header = 0;
    #process non-header lines (if needed) 
    #...
  }
}
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