In Perl it is very easy to 'slurp' files, and it is a really bad habit to get into. In this case one of the files needs to be slurped so try to make it the smallest. There is actually a hidden loop in the code below - the grep, but if you understand grep, then it does make the code much clearer.

As a Perl beginner you should note that strictures should always be used (use strict; use warnings;), file opens should use the three parameter version and are checked, and that some checking (but not as much as there ought to be) is made on the input data.

use warnings;
use strict;

my $List1 = <<DATA;
.2
.3
.4
DATA

my $List2 = <<DATA;
.4
.5
.6
DATA

open my $in1, '<', \$List1 or die "Failed to open List1: $!";
my @data1 = grep {chomp; length} <$in1>;
close $in1;

my $hitCount = 0;

open my $in2, '<', \$List2 or die "Failed to open List2: $!";
while (<$in2>) {
    chomp;
    next unless length;

    my $data2 = $_;
    $hitCount += grep {abs ($data2 - $_) <= 0.2} @data1;
}

close $in2;

print "Found $hitCount matches\n";
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Prints:

Found 6 matches
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