An even better way to report an error would be croak() from the Carp module. This provides a stack trace. While that doesn't matter in a very simple linear program, things change quite a bit as soon as you write a single function that could be called from multiple places.

#!/usr/bin/env perl

use v5.36;
use strict;
use warnings;

use Carp;

# --------- make demo file
open(my $ofh, '>', 'exists.dat') or croak($!);
for(1..3) {
    print $ofh "BLA\n";
}
close $ofh;
#---------

print "exists.dat has ", countLines('exists.dat'), " lines\n";

my $linecount = 0;
foreach my $fname (qw[exists.dat doesnotexist.dat]) {
    $linecount += countLines($fname);
}

sub countLines($fname) {
    my $linecount = 0;
    open(my $ifh, '<', $fname) or croak("$!");
    while((my $line = <$ifh>)) {
        $linecount++;
    }
    return $linecount;
}
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When we run it, we get a nice backtrace:

$ perl carptest.pl 
exists.dat has 3 lines
No such file or directory at carptest.pl line 26.
    main::countLines("doesnotexist.dat") called at carptest.pl line 21
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