If you have a file with a known encoding, for instance koi8-r, just use

open( $fh, '< :encoding(koi8-r)', "in_file");
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and it should work fine under utf8::all.

And i want the scripts to: a) warn me when i open a file that contains non-utf characters rather than die at it;

I don't think it's possible to do it "when the file is opened" because the error arises when some non-Unicode utf8 sequence is read into Perl's internals. One way to do it is to use eval while reading the file line-by-line and trap the error. Here is some code which does that, trying first in utf8 and if that fails to be valid, warns and retries with koi8-r.

 
use strict; 
use warnings; 
use utf8::all;

open(my $fh, '<', "in_file") or die "cannot open in_file: $!";
eval { process_file_by_line() }; 

if ( $@ =~ /does not map to Unicode/ ) {
    warn $@;
    print "...trying encoding koi8-r instead of utf8\n\n";
    close $fh;
    open( $fh, '< :encoding(koi8-r)', "in_file") or die "cannot open i
+n_file: $!";
    process_file_by_line();
} 
elsif ( $@ ne '' ) {
    die $@;   # bail out on other eval errors
}

sub process_file_by_line {
    while ( <$fh> ) {
        print;
        # whatever else...
    }
}
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