The difference is context. In scalar context, a regex usually returns the number of matches or the number of substitutions, which is not what you want here. Adding parentheses creates a list context in which a regex will usually return something closer to what you want here.
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Adding parentheses creates a list context ...
This is either terminal nitpicking on my part or a confession of total ignorance, but I thought that for substitution, the
($x = $y) =~ s///;
form (case B below) worked by circumventing normal operator precedence and making the value, normally a scalar, which is bound by the =~ operator into an assignment expression, which causes the substitution result to be assigned to the assignment lvalue. I.e., list context does not come into play at all. In the
$x = $y =~ s///;
statement (case C), the =~ operator has precedence over = and the s/// return value, the number of substitutions in any context, is assigned. (Somewhere in my Perlish consciousness, I have the idea that substitution normally builds its result in a temporary variable, which is then copied to the bound scalar upon successful completion of the substitution. If an assignment expression is detected, the temp result is simply copied to a different destination.)
Of course, for a m// match operation (case A below), list context can be very important.
c:\@Work\Perl>perl -wMstrict -le
"my $y = '2017-01-29 11:30:07.370';
my $x;
;;
($x) = $y =~ m{ (\d{4}) - (\d{2} - \d{2}) \s+ (\d{2} : \d{2}) .* }xms
+;
print qq{A: x '$x' y '$y'};
;;
($x = $y) =~ s{ (\d{4}) - (\d{2} - \d{2}) \s+ (\d{2} : \d{2}) .* }
{$2-$1 $3}xms;
print qq{B: x '$x' y '$y'};
;;
$x = $y =~ s{ (\d{4}) - (\d{2} - \d{2}) \s+ (\d{2} : \d{2}) .* }
{$2-$1 $3}xms;
print qq{C: x '$x' y '$y'};
"
A: x '2017' y '2017-01-29 11:30:07.370'
B: x '01-29-2017 11:30' y '2017-01-29 11:30:07.370'
C: x '1' y '01-29-2017 11:30'
Give a man a fish: <%-{-{-{-<
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[d/l]
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