#1
$_ = $digit;
s/.*(\d).*/$1/; # change everything to the captured digit
$digit = $_;

#2
$_ = $digit;
m/(\d)/ && $digit = $1 # change $digit to be the captured digit

#3
$digit =~ m/(\d)/ && $digit = $1

#4
$digit =~ s/.*(\d).*/$1/;