• Don't write it that way to begin with . . . :)
• perltidy as was mentioned before
• This is Perl, not C. Explicitly testing against 0 for truth of a boolean (by which I mean solely true/false; if you're (say) encoding state that's a different proposition) is kinda klunky; just use the variable in a boolean sense and lose the == 1 and == 0 stuff
• Consider refactoring by decomposing the parts, if not into separate subs at least into separate conditional variables (e.g. have my $three_arg_interval_model = scalar @ARGV >= 3 && $oneDayFlag == -1;, and your conditional will read a bit better if( ( $wrongDateFlag and ( $three_arg_interval_model or $two_arg_oneday_model ) ) or $three_arg_wrong_date ) { ... } (and of course you can combine that first and clause into another variable too if you really want))

Update: And if you're looking to trim verbiage those explicit scalar prefixes are technically extraneous (i.e. just @ARGV >= 3 works just as well), but that's really more a personal preference (not that I don't do it myself from time to time).

If you also are looking for hints on coding style, the O'Reilly book 'Perl Best Practices' might be something for you.

My advice on your code sample would be to use 'and' instead of '&&', less errorprone and better to read IMHO. And group the comments together to say what the if clause does altogether instead of commenting on each single term

Another option is:

my $doit;
if ($wrongDateFlag) {
  $doit = 1 if scalar @ARGV >= 3;
} else { # end date is correct
  if ($oneDayFlag == -1) {
    $doit = 1 if scalar @ARGV >= 3;
  } else {
    $doit = 1 if scalar @ARGV >= 2;
  }
}
if ($doit) {
 ...
}
[download]

Minor tweaks...

my $doit;
if ($wrongDateFlag) {
  $doit = 1 if scalar @ARGV >= 3;
} else { # end date is correct
  $doit = 1 if scalar @ARGV >= (($oneDayFlag == -1) ? 3 : 2 );
}
if ($doit) {
 ...
}
[download]

Now I'm getting silly...

my $doit = scalar @ARGV >= ($wrongDateFlag ?  3 : (($oneDayFlag == -1)
+ ? 3 : 2 ));

if ($doit) {
 ...
}
[download]

I haven't tested whether this matches your logic, but it should be close, anyway:

sub body { ... your code ...}

SWITCH: {
    body, last SWITCH if $wrongDateFlag and $#ARGV >= 2;
    body, last SWITCH if not $wrongDateFlag and $#ARGV >=2 and $oneDay
+Flag == -1
;
    body, last SWITCH if not $wrongDateFlag and $#ARGV >=1 and $oneDay
+Flag >= 0;
    $felloff == 1;
}
[download]

UPDATE:

I forgot $#ARGV returns one less than scalar @ARGV so I fixed that. Some would probably fault me for using a switch statement where each path calls the same code but I like the resulting look. Cleaner might be.

sub body { ... your code ...}

body if $wrongDateFlag and $#ARGV >= 2
        or not $wrongDateFlag and $#ARGV >=2 
        or not $wrongDateFlag and $#ARGV >=1 and $oneDayFlag >= 0;
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UPDATE:

removed some vestigial semicolons.

if( @ARGV >= ($wrongDateFlag ? 3 : $oneDayFlag >= 0 ? 2 : 3) )  {
}
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if( @ARGV >= ($wrongDateFlag || $oneDayFlag == -1 ? 3 : 2) ) {
}
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use Getopt::Long;

my ($oneDayFlag, $someDate);

my $result = GetOptions (
        'oneday=i'=>\$oneDayFlag,
        'somedate=s'=>\$someDate
    );

...
[download]