#!/usr/bin/perl
use strict;
use warnings;

my $fn = "/tmp/test.dat";

open  (DATAFILE, ">$fn") or die "Could not open file $fn for writing";
print DATAFILE "A B C\n";       # NO 
print DATAFILE "AA BB CC\n";    # YES
print DATAFILE "AAA BBB CCC\n"; # YES
close (DATAFILE);

open (READER, "<$fn") or die "Could not open file $fn for reading";

my $input;
while ($input = <READER>)
{
  # perldoc perlre for syntax of regex
  if ( $input =~ /A.*B..C/ )
  {
    print "MATCH $input";
  }
}

close (READER);