Beware of what you ask for. The following script removes every line that only contains valid UTF-8.

#!/usr/bin/perl
use strict;
use warnings;
use Encode qw( decode );
while (<>) {
   print if !eval { decode('UTF-8', $_, Encode::FB_CROAK); 1 };
}
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Usage:

• From a file: remove_utf8_lines.pl infile > outfile
• From STDIN: remove_utf8_lines.pl < infile > outfile
• In-place: perl -i.bak remove_utf8_lines.pl file

A better solution might be to convert the lines to another encoding.

#!/usr/bin/perl
use strict;
use warnings;
binmode(STDIN,  ':encoding(UTF-8)');
binmode(STDOUT, ':encoding(iso-latin-1)');
print while <>;
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Same usage as the original program.