A range operator would give you an arguably elegant rewrite:

@$msg = grep {(m/\QContent-Transfer-Encoding: 7bit\E/i .. /^\s*$/) ne 
+'2E0'} @$msg;
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or, to use the splice and short-circuiting:

my $i = 0;
for (@$msg) {
  if ((/\QContent-Transfer-Encoding: 7bit\E/ .. /^\s*$/) eq '2E0') {
    splice(@$msg, $i, 1);
    last;
  }
  ++$i;
}
[download]