examples/files/open_with_if.pl
#!/usr/bin/perl
use strict;
use warnings;

my $filename = "input.txt";
if (open my $in, "<", $filename) {
    # do your thing here
    # no need to explicitly close the file
} else {
    warn "Could not open file '$filename'. $!";
}
# here the $in filehandle is not accessible anymore

A more Perlish way to open a file and exit with error message if you could not open the file:

examples/files/open_with_die.pl
#!/usr/bin/perl
use strict;
use warnings;

my $filename = "input.txt";
open(my $fh, "<", $filename) or die "Could not open file '$filename'. $!";
# do your thing here
close $fh;