use strict;
use warnings;

my @strings = ('String A', 'String B', 'String C', 'String D', 'String E');

my %string_handles = map { open my $fh, '>', "$_.log" or die "Couldn't create '$_.log': $!"; $_ => $fh }
    @strings;

open (my $in_fh, '<', 'input.txt') or die "Couldn't open input file!\n";

local $/ = '';
while (my $paragraph = <$in_fh>) {
    while (my ($string, $out_fh) = each %string_handles)
    {
        next unless index($paragraph, $string) >= 0;
        print {$out_fh} $paragraph;
    }
}

close $in_fh;