$ENV{P2KTMP} = '/foo/bar';
my $consolidatedPath = '$P2KTMP/data';
my $unsafe = '$P2KTMP/data';

# Method 1
my @ALLOWED_VARS = qw/ P2KTMP FOO BAR /;
my $VAR_RE = join '|', map {quotemeta} @ALLOWED_VARS;
$consolidatedPath =~ s/\$($VAR_RE)\b/$ENV{$1}/g;

# Method 2
chomp( $unsafe = `echo $unsafe` );

print "$consolidatedPath\n";
print "$unsafe\n";

__END__
/foo/bar/data
/foo/bar/data