my $var;
if ($string =~ /$capture_regex/) {
    $var = $1;
}

##</code><code>##

my ($var) = $string =~ /$capture_regex/;

##</code><code>##

next unless $string =~ /$capture_regex/;
my $var = $1;

##</code><code>##

my $var;
if ($string =~ /$capture_regex/) {
    $var = $1;
}
else {
    # ... issue warning, make a log entry, or whatever ...
    next;
}

##</code><code>##

$ grep soft /usr/share/dict/words | wc -l
30
$ grep ^soft /usr/share/dict/words | wc -l
22

##</code><code>##

/ \b (?: soft | softer | softest ) \b /ix

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;

my $date_re = qr{(?x: ^ <time \s datetime= ( \d{4} - \d{2} - \d{2} ) )};
my $want_re = qr{(?sx: [#][#] ( .*? ) [#][#] )};
my $soft_re = qr{(?ix: \b ( soft ) )};

my %count_for_date;

for (values %{ get_corpus() }) {
    my ($date) = /$date_re/;
    my ($want) = /$want_re/;
    $count_for_date{$date}++ while $want =~ /$soft_re/g;
}

# For testing only:
use Data::Dump;
dd \%count_for_date;

sub get_corpus {
    my %corpus = (
        fileA => "<time datetime=2017-09-01...
            soft soft
            soft
            ##... soft soft ... soft ...##
            hard
            soft
        ",
        fileB => "<time datetime=2017-09-01...
            soft
            ##... hard ...##
            soft
        ",
        fileC => "<time datetime=2017-09-01...
            ##... softball ...##
        ",
        fileD => "<time datetime=2017-09-02...
            ##... semisoft ...##
        ",
        fileE => "<time datetime=2017-09-03...
            ##... soft softer softest softly soften softner ...##
        ",
        fileF => "<time datetime=2017-09-04...
            ##
                soft softer softest 
                Soft Softer Softest 
                softly soften softner 
                Softly Soften Softner 
            ##
        ",
    );

    return \%corpus;
}

##</code><code>##

{ "2017-09-01" => 4, "2017-09-03" => 6, "2017-09-04" => 12 }

##</code><code>##

$count_for_date{$date} ||= 0;

##</code><code>##

{ "2017-09-01" => 4, "2017-09-02" => 0, "2017-09-03" => 6, "2017-09-04" => 12 }