my $row = 'We_need_feed';

#my ($last) = $row =~ /[^_]$/g; #match we
# [^_] matches everything except underscore, but only one character; 
# Plus you've anchored it to the last character before the end of the string,
# so it will always and only match the last character of the string; 
# unless it is an underscore, in which case it fails to match at all.


#my ($last) = $row =~ /^\w?[^_]/g;#match feed
# ^\w? matches a single non-whitespace character at the start of the string, 
# if the first character isn't whitespace, otherwise it matches nothing.
# [^_] matches a single non-underscore as above. 
# Ie. You matched two characters "We"

my ($last) = $row =~ /[need]+$/g ;
# [need]+ matches a one or more characters, so long as they are either 'd' or 'e' or 'n'; other wise nothing
# but the $ means only at the of the line.
# so this matched the 3-char string 'eed'.