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 in_file: $!";
    process_file_by_line();
} 
elsif ( $@ ne '' ) {
    die $@;   # bail out on other eval errors
}

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