while (my $line = <INFILE>) {
    $line =~ s/(\S+)/$dict{$1} || $1/eg;
    print $line;
}

##</code><code>##

    $line =~ s/(\S+)/defined $dict{$1} ? $dict{$1} : $1/eg;

##</code><code>##

    $line =~ s|(\S+)|$dict{$1} // $1|eg;