My first approach would be a lazy one; just apply a regex to the single line representing the concatentation of all lines from the file ...

use strict;
use warnings;
use File::Basename;
use IO::File;

my $iam = basename $0;
(my $fname = shift) or die "syntax:  $iam  <file>\n";
my $fh = new IO::File($fname) or die "$iam:  can't read $fname ($!)\n"
+;

undef $/;                                      # Read file into a sing
+le line
(my $line = <$fh>) =~ s/\n(\s*\n){2,}/\n\n/g;  # Delete extra blank li
+nes
printf "%s\n", $line;                          # Show the result
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You could also do it by reading a single line at a time, and only printing a single blank line for each group encountered; something like ...

use strict;
use warnings;
use File::Basename;
use IO::File;

my $iam = basename $0;
(my $fname = shift) or die "syntax:  $iam  <file>\n";
my $fh = new IO::File($fname) or die "$iam:  can't read $fname ($!)\n"
+;

my $b_prev_blank = 0;    # Nonzero each time the previous line was bla
+nk
my $b_this_blank = 0;    # Nonzero each time the current line is blank
while (my $line = <$fh>) {
    chomp $line;
    $b_this_blank = ($line =~ /^\s*$/);
    (!$b_prev_blank or !$b_this_blank) and printf "%s\n", $line;
    $b_prev_blank = $b_this_blank;
}
[download]