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my $C='((w)hite|(b)lack)';

...
white "princepawn"
foo
black "princepawn"

sub popstr   { substr $_[0], -1, 1, '' } # or chop
sub shiftstr { substr $_[0],  0, 1, '' }
...
pushstr $string, $foo;      # estint
unshiftstr $string, $bar;   # gestint
print "$string\n";

sub popstr   { $_[0] =~ s/(.)\z//s; $1 }
sub shiftstr { $_[0] =~ s/^(.)//s;  $1 }
...
pushstr $string, $foo;      # estint
unshiftstr $string, $bar;   # gestint
print "$string\n";

2;0 juerd@ouranos:~$ perl -e'undef christmas'
Segmentation fault
2;139 juerd@ouranos:~$