You're talking about undefined $/. Reading a whole file at once is called "slurping the file" or "slurping the file's content".

my $file; { local $/; $file = <>; }
$file =~ s/(?<!\n)\n(?!\n)//g
print $file;
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Usage for above:

perl script.pl infile > outfile

perl -i.bak script.pl file   # in-place
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As a one-liner:

perl -0777pe's/(?<!\n)\n(?!\n)//g' infile > outfile

perl -i.bak -0777pe's/(?<!\n)\n(?!\n)//g' file
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