in reply to difference in regex

You will find the answer to your question in "Regexp Quote-Like Operators" in perlop - basically, different regex operations have different return values in different contexts. See also perlretut for a tutorial.

Operation Context () Capturing
Return Value on Match
(and notes on behavior)
Return Value on Failure Example
m// scalar - true false
my $x = "foobar"=~/[aeiou]/; # => $x is true my $y = "foobar"=~/[xyz]/; # => $y is false
m//g scalar - true
(each execution of m//g finds the next match,
see "Global matching" in perlretut)
false if there is no further match
my $str = "foobar"; my $x = $str=~/[aeiou]/g; # matches first "o" => $x is true, pos($str) is 2 $x = $str=~/[aeiou]/g; # matches second "o" => $x is true, pos($str) is 3 $x = $str=~/[aeiou]/g; # matches "a" => $x is true, pos($str) is 5 $x = $str=~/[aeiou]/g; # no more matches => $x is false, pos($str) is undef
m// list no the list (1) the empty list ()
my ($x) = "foobar"=~/[aeiou]/; # => $x is 1
m//g list no a list of all the matched strings, as if there were parentheses around the whole pattern the empty list ()
my ($x,$y,$z) = "foobar"=~/[aeiou]/g; # => $x is "o", $y is "o", $z is "a"
m// list yes a list consisting of the subexpressions matched by the parentheses in the pattern, that is, ($1, $2, $3...) the empty list ()
my ($x,$y) = "foobar"=~/([aeiou])(.)/; # => $x is "o", $y is "o"
m//g list yes a list of the substrings matched by any capturing parentheses in the regular expression, that is, ($1, $2...) repeated for each match the empty list ()
my ($w,$x,$y,$z) = "foobar"=~/([aeiou])(.)/g; # => $w is "o", $x is "o", $y is "a", $z is "r"
s/// - - the number of substitutions made false
my $x = "foobar"; my $y = $x=~s/[aeiou]/x/g; # => $y is 3
s///r - - a copy of the original string with substitution(s) applied
(available since Perl 5.14)
the original string
my $x = "foobar"=~s/[aeiou]/x/gr; # => $x is "fxxbxr"

In this table, "true" and "false" refer to Perl's notion of Truth and Falsehood. Remember not to rely on any of the capture variables like $1, $2, etc. unless the match succeeds!

In my $foo = "bar"=~/a/;, the right-hand side of the assignment ("bar"=~/a/) is in scalar context. In my ($foo) = "bar"=~/a/; or my @foo = "bar"=~/a/;, the right-hand side is in list context. That's why, in your example, you need those parens in ($value): because you want the matching operation to return the contents of the capture group.

Note that your expressions can be slightly simplified, not all the parens you showed are needed:

my ($value) = $row =~ /.*,(.*)/; # and $row =~ s/,[^,]*$//;

A few additional comments on your code:

Update: Added s///r to the table and added a few more doc links. A few other edits and updates. 2019-02-16: Added "Return Value on Failure" column to table, and a few other small updates. 2019-08-17: Updated the link to "Truth and Falsehood".