You haven't asked a question, nor explained what isn't working, so this is a guess.

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 s
+tring, 
# if the first character isn't whitespace, otherwise it matches nothin
+g.
# [^_] 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'.
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• To match the first word, delimited by underscore (_), you could use /^[^_]+/.
• To match the last word delimited by underscore, you could use /[^_]+$/.
• To match the word 'need', somewhere in the line you could use /need/.

You'll get better answers if you tell us what you need to know in words as well as code. Relying on us to read between the lines of your non-functional code and comments to extract the meaning and question, is unlikely to get the best answers. Indeed, If I wasn't bored out of my mind waiting for a process that has been running for 50+ hours to finish, I wouldn't have bothered answering your lazy post at all.