# option 1
print "Enter the name for the output file:\n"; 
my $outfile = <STDIN>;
chomp $outfile; #parens not needed

# option 2
my $outfile = 'output.txt';

# option 3a
my $outfile = $firstfilename.'.out';

# option 3b - using $0 aka the current program filename

use File::Basename qw(basename);

my $outfile = (basename $0).'.out';