c:\@Work\Perl\monks>perl -wMstrict -le
"my $ppi_pm_seq = 'ACTGCCT';
 ;;
 my $n = 3;
 ;;
 my $extraction =
 my ($before, $mid, $after) =
 $ppi_pm_seq =~ m{ \A (.{$n}) (.) (.{$n}) \z }xms;
 ;;
 die qq{no extraction from '$ppi_pm_seq'} unless $extraction;
 ;;
 my ($ppi_pm_id, $ppi_mm_id, $mpi_pm_id, $mpi_mm_id) =
   map qq{$before$_$after},
   $mid, grep $_ ne $mid,
   qw(A T C G)
   ;
 ;;
 print qq{'$_'} for $ppi_pm_id, $ppi_mm_id, $mpi_pm_id, $mpi_mm_id;
"
'ACTGCCT'
'ACTACCT'
'ACTTCCT'
'ACTCCCT'

##</code><code>##

my $s = 'ATCGxTGAC';
my $foreign = $s =~ tr/ATCG//c;
print qq{foreign character in '$s'} if $foreign;