basic problem I don't get!

two possible causes:
you should predeclare %FORM outside of the foreach loop, or it will only be available inside of it (i probably wrong about the necessity of this one, but am used to stricter coding rules than normal cgi's).
in the foreach loop, you should say '$FORM{$name} ...' instead of '$FORM{$get_page} ...'. this is the spot that's probably the cause. when it sets the value, it's setting it to the entry in %FORM keyed by the value of $get_page(which is nothing at this point), and NOT setting it to $get_page. i'm not quite sure, because if this is the case, it would have always defaulted to case 'three', which you say it didn't. in any case, i made the if conditions stricter as well to prevent that particular happenstance. now, for the 'if' conditionals at the bottom, you'll need to say 'if ($FORM{get_page} eq 'one')'.
so, here's what the main part will look like:[tt]

read(STDIN,$buffer,$ENV{'CONTENT_LENGTH'});
%FORM = ();
@pairs=split(/&/,$buffer);
foreach $pair(@pairs)
{
($name,$value)=split(/=/,$pair);
$value=~tr/+//;
$value=~s/%([a-fA-f0-9][a-fA-f0-9])/pack("C",hex($1))/eg;
$FORM{$name}=$value;
}

if ($FORM{get_page} eq "one&quot {
print "Location:

http://127.0.0.1/under_construction.html\n\n";

}
elsif ($FORM{get_page} eq "two&quot {
print "Location:

http://127.0.0.1/help.html\n\n";

}
elsif ($FORM{get_page} eq "three&quot {
print "Location:

http://127.0.0.1/server.html\n\n";

}
[/tt]
next time this happens, try using 'perl -w' and you'll get warning messages that may lead you to finding the problem. "If you think you're too small to make a difference, try spending a night in a closed tent with a mosquito."