if (<FH>) VS while (<FH>)

If (<FH>) ::: if doesn't not fill the $_ variable with contents of
files first line automatically.
While (<FH>) ::: while do fill the $_ to content of the files first
line automatically.

i.e
open FH, "name.txt" or die "cant open";
if (<FH>){
print "$_";
}

prints nothing
If we change if to while $_ is set to first line of the file... how
can this happen

.