regex - Why the following variations of s/g from command line are wrong? -

i have small file follows:

$ cat text.txt vacation cat test 

this command substitutes occurrences of cat cat correctly wanted:

$ perl -p -i -e ' s/cat/cat/g; ' text.txt  

but why following 2 mess file up?
following deletes contents of file

$ perl -n -i -e '   $var = $_;   $var =~ s/cat/cat/g;   ' text.txt   

and 1 not substitution correctly

$ perl -p -i -e ' $var = $_; $var =~ s/cat/cat/g; ' text.txt  $ cat text.txt cation cat test 

why? messing here?

-p prints out each line automatically (the contents of $_, contains current line's contents), re-populates file (due -i flag in use), -n loops on file -p does, doesn't automatically print. have yourself, otherwise overwrites file nothing. -n flag allows skip on lines don't want re-insert original file (amongst other things), whereby -p, you'd have use conditional statements along next() etc. achieve same result.

perl -n -i -e '   $var = $_;   $var =~ s/cat/cat/g; print $var;   ' text.txt   

see perlrun.

in last example, -p automatically print $_ (the original, unmodified line). doesn't auto-print $var @ all, in case, you'd have print $var in example above, you'd both original line, , modified 1 printed file.

you're better off not assigning $_ if you're doing overwriting file. use is. eg. (same first example):

perl -p -i -e '    s/cat/cat/g;   ' text.txt