I can't reproduce this problem. I tried with perl 5.8.8 and 5.10.0 with this script:

#!/usr/bin/perl
use strict;
use warnings;

my  $swiss = 'P07445';
warn "swiss='$swiss'\n";
my @rer;
$rer[0] = $swiss =~ /^\p{IsAlpha}/;
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Most likely some XS or Inline::C module (or an interpreter bug) corrupts your program's memory, and then it dies at an otherwise harmless operation.

Please try to reproduce the problem with a minimal but complete example (i.e. it crashes, but uses as little modules and code as possible) - I don't think the example you gave us crashes on your perl, unless it's executed in the context of another program.