my %strings;

while (<FILE1>) {
    chomp;
    $string{$_} .= '1';  # if same data appears three times, hash value is "111";
}

while (<FILE2>) {
    chomp;
    $string{$_} .= '2';  # same as above, but with "2" instead of "1"
}

# get hash keys (lines) that occur in both files:

my @common = grep { $strings{$_} =~ /12/ } sort keys %strings;

# report findings:

for my $key ( @common ) {
    my ( $n1, $n2 ) = ( $strings{$key} =~ /(1+)(2+)/ );
    printf("%s found %d times in file1, %d times in file2\n",
           $key, length($n1), length($n2));
}

# you can also pick out strings unique to file1 (/1$/)
# and/or strings unique to file2 (/^2/), along with their
# frequency of occurrence.  This also scales fairly well to
# handling three or more files.