my $tempport;                      # declare var
if(  $args{DOUBLE_BYTE} =~ /y/i )  # try to match with a regex
                                   # regex is just looking for 'y'
                                   # the /i means case-insensitive
{
  $tempport = DB_CATPORT;         # Set to DB_CATPORT
}
else
{
  $tempport = SB_CATPORT;         # Set to SB_CATPORT
}
[download]

(though i guess DOUBLE_BYTE could be a constant, in which case the key is really the constant's value.)

Actually, it's not:

use constant a => 'b';
my %foo;
$foo{a} = "x";
print keys %foo;
__END__
a
[download]

By that logic, it should print 'b' instead. Remember, "constants" in Perl are really just subroutines, not C-style macros. The rule that a bareword as a hash key is always treated as a string applies whether or not there's a subroutine or constant with that name defined.

my $tempport;

if ( $args{DOUBLE_BYTE} =~ /y/i )
{
  $tempport =  DB_CATPORT;
}
else
{
  $tempport = SB_CATPORT;
}
[download]

without knowing the rest off the code, ussumtions have to be made, but my guess:

if $args{DOUBLE_BYTE} contains a 'y' character (case doesn't matter), $tempport is set to the constant value DB_CATPORT.
Otherwise, $tempport is set to the constant SB_CATPORT.

the following code does the same:

my $tempport;
if ( $args{DOUBLE_BYTE} =~ /y/i ) {
    $tempport = DB_CATPORT
}
else {
    $tempport = SB_CATPORT
}
[download]

It's the same as:

my $tempport;
if ($args{DOUBLE_BYTE} =~ /y/i) {
  $tempport = DB_CATPORT;
} else {
  $tempport = SB_CATPORT;
}
[download]