With 5.14+, the  /r modifier can be used with  s/// substitution to achieve non-mutating substitution with return of a copy of the string (although the intermediate array creation by map noted by BrowserUk will, I think, still occur). See  s/// in the Regexp Quote-Like Operators section of perlop:

If the "/r" (non-destructive) option is used [in s///] then it runs the
substitution on a copy of the string and instead of returning
the number of substitutions, it returns the copy whether or not
a substitution occurred. The original string is never changed ... 
>perl -wMstrict -le
"my @list= qw(first_string  second_string  THIRD  fourth_string);
 my @newlist = map s/_string$//r, @list;
 printf qq{'$_'  } for @list;
 print '';
 printf qq{'$_'  } for @newlist;
"
'first_string'  'second_string'  'THIRD'  'fourth_string'
'first'  'second'  'THIRD'  'fourth'

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In reply to Re: using map and swap (aka substitute) by AnomalousMonk
in thread using map and swap (aka substitute) by DamianKaelGreen

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