my %respell = ( 'ze' => "\x{017e}e", ... );

##</code><code>##

use Unicode::Normalize;

my %respell;
open( INFO, "<:utf8", "cetnosti" ) or die "cetnosti: $!";
while (<INFO>) {
    next unless ( /[^[:ascii:]]/ ); # skip words that are all-ascii

    my ( $word, $freq ) = split;

    my $ascii_word = NFD( $word );  # break accented letters into letter, diacritic
    $ascii_word =~ s/[^[:ascii:]]+//g; # delete diacritics

    $respell{$ascii_word} = $word;
}
close INFO;

##</code><code>##

open( INPUT, "<:utf8", "input" ) or die "input: $!";
open( OUTPUT, ">:utf8", "respelled" ) or die "respelled: $!";

while (<INPUT>) {
    my $outstr = '';
    for my $tkn ( split /(\s+)/ ) {
        if ( exists( $respell{$tkn} )) {
            $tkn = $respell{$tkn};
        }
        $outstr .= $tkn;
    }
    print OUTPUT $outstr;
}