use strict;
use warnings;

# no need for your comment! it is clear!!
my $outfilename = "pot_temp.txt";  # define an output filename

# always check the return values of file operations !!!
# there a bunch of syntax'es for this, the main thing is to do it!

#open(TFILE,">$outfilename") || die ("unable to open $outfilename $!");
# below I don't use TFILE, this is just a test to stdout

#there are shorter ways to do this, but this is staightforward. 
#Maybe these are pressure increments? And I have wrong name?
#whatever, this is one way to do it...

my @temp_increments;
for (my $i=500; $i<=900 ; $i += 100)
{
   push @temp_increments, $i;
}

#my @temp_increments = qw( 500, 600, 700, 800, 900);
#would be equivalent for example

print "What is the temperature (K)?:";
my $T = <STDIN>;
chomp ($T);

print "T results are:\n";
foreach my $temp (@temp_increments)
{
  my $theta = $T*((1013/$temp)**(287/1004));
  print "temp = $temp  theta=$theta \n";
}

#Example output......
#C:\TEMP>perl temp.pl
#What is the temperature (K)?:26
#T results are:
#temp = 500  theta=31.8147304436198
#temp = 600  theta=30.1990838700515
#temp = 700  theta=28.8972548674746
#temp = 800  theta=27.8150092740013
#temp = 900  theta=26.8940943585568