m modifier treats the string as a set of multi line. In your regular expression "." (dot) matches a character other than \n (newline), because it treats the string as a multiline.

s modifier is very simple, it takes the whole string (doesnt care about how many \n), and consider as a single long string.

my $string;

$string="PERL is an Acronym of practical extraction and report languag
+e\ni like it very much";

# here "." is as usual i.e any single character.(including \n) so it m
+atches and returns 1. 
print $string =~ /language.i/s;  

# here "." matches any character other than \n, but in our string lang
+uage followed by \n is there. so it doesnt return 1. its false. 
print $string =~ /language.i/m;

# here \n followed by i will match so returns true. 
print $string =~ /language\ni/m;

# As simple s modifier consider . as a single character. that could be
+ anything... returns true...
print $string =~ /language\ni/s;
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