if (! open my $in_fh, '<', $in ) {
    #      ^^ <- HERE
    die "Can't open '$in': $!";
}

##</code><code>##

if (! open my $fh, '<', $file){
    die $!;
}

# file handle already closed if you try to access here

##</code><code>##

{
    open my $fh, '<', $file or die $!;

    # can use the fh here
}

# but not here, fh auto-closed in the block

##</code><code>##

my $fh;

if (! open $fh, '<', $file){
    die $!;
}

# you can successfully use fh here...
# it'll only close if you "close $fh or die $!;", or when the file (program) exits

# the above example has the same effect of doing the below example, but this is 
# easier as you don't have a bunch of variables declared at the top of your file.

# the entire idea is to keep variables declared, defined as close to
# the place you intend to use them

open my $fh, '<', $file or die $!;