# regex /.{0,3}X.{0,3}/
#
# string       matched part

123X123         123X123
12X123          12X123
1X123           1X123
X123            X123
X123456         X123


##</code><code>##

# regex /.{3}X.{3}/
#
# string       matched part

123X123         123X123
12X123          -no match-
1X123           -no match-
X123            -no match-
X123456         -no match-


##</code><code>##

# open my $fh, '<', $file_path or die "unable to read $file_path"

# list context: every line goes in the array
my @all_lines = <$fh>;


# scalar context: just next line goes into a scalar (<> acts as an iterator here)
my $line = <$fh>;

# so to read a file one line at time:
while (defined( my $line= <$fh>)) {