How to replace a string in an existing file in Perl?

Question

I want to replace word \"blue\" to \"red\" in all text files named as 1_classification.dat, 2_classification.dat and so on. I want to edit the same file so I tried this code

Answer 1

$_='~s/blue/red/g';

Uh, what??

Just

s/blue/red/g;

or, if you insist on using a variable (which is not necessary when using $_, but I just want to show the right syntax):

$_ =~ s/blue/red/g;