The following does what it seems you want and cleans up your original code somewhat. Note however that modules such as HTML::TreeParser are generally much better (more reliable and easier to maintain) tools for extracting the raw data from an HTML page such as the source of this data seems to be.

use strict;
use warnings;

my $lat;
my $long;

# First seek location line
while (<DATA>) {
     next unless /(-?\d+(?:\.\d*)?)\s+ (-?\d+(?:\.\d*)?)\s+ \d+\s \d+/
+x;
     
     ($lat, $long) = ($1, $2);
     last;
}

print "$lat, $long\n";

# Skip to data lines
while (<DATA>) {last if /^-+$/};
while (<DATA>) {last if /^-+$/};

while (<DATA>) {
     my ($LEV, $PRES, $HGHT,  $TEMP, $DEWP, $RH, $DD, $WETB, $DIR, $SP
+D) =
          split ' ';

     last unless defined $SPD;     
     print "$PRES, $HGHT, $TEMP, $DEWP, $DIR, $SPD\n";
}

__DATA__
<TITLE>Plymouth State RAOB Thermodynamic Diagram/Data</TITLE>
</center><pre>
Miami Intl Airp FL US KMIA 1 25.82 -80.28 4 72202

Date: 0000Z 24 AUG 05
Station: KMIA
WMO ident:  72202
Latitude:   25.82
Longitude: -80.28
Elevation:   4.00
----------------------------------------------------------------------
+---------
LEV PRES  HGHT  TEMP  DEWP  RH  DD   WETB DIR SPD THETA THE-V THE-W TH
+E-E   W
     mb     m     C     C    %   C     C  deg knt   K     K     K     
+K    g/kg
----------------------------------------------------------------------
+---------
SFC 1012     4  29.8  23.8  70  6.0  25.3 100  10 301.9 305.3 298.1 35
+7.0 18.65
  1 1000   113  28.8  23.8  74  5.0  25.1  95  12 302.0 305.4 298.2 35
+7.7 18.88
  2  961   466  25.4  22.3  83  3.1  23.2  93  15 302.0 305.3 297.7 35
+4.9 17.91
  3  925   802  23.2  19.4  79  3.8  20.5  95  15 303.0 305.9 296.4 34
+9.0 15.51
  4  850  1536  19.0  14.0  73  5.0  15.7  95  14 306.0 308.3 294.7 34
+1.8 11.91
  5  769  2390  14.2   8.2  67  6.0  10.5  96  11 309.8 311.4 293.6 33
+7.0  8.91
  6  753  2568  12.4  10.5  88  1.9  11.2  97  11 309.7 311.7 294.8 34
+2.2 10.65
  7  737  2748  11.8   6.8  71  5.0   8.8 100  11 310.9 312.5 293.6 33
+6.9  8.44
  8  700  3178   9.4   4.4  71  5.0   6.5 110  11 312.9 314.3 293.4 33
+6.3  7.51
</pre>
[download]

Prints:

25.82, -80.28
1012, 4, 29.8, 23.8, 100, 10
1000, 113, 28.8, 23.8, 95, 12
961, 466, 25.4, 22.3, 93, 15
925, 802, 23.2, 19.4, 95, 15
850, 1536, 19.0, 14.0, 95, 14
769, 2390, 14.2, 8.2, 96, 11
753, 2568, 12.4, 10.5, 97, 11
737, 2748, 11.8, 6.8, 100, 11
700, 3178, 9.4, 4.4, 110, 11
[download]