This solution excludes empty/whitespace lines:

use strict;
my $line_number = 0;
while (<>) {
    chomp;
    next if (m/^\s*$/);
    ++$line_number;
    print $line_number, ': ', $_, "\n";
}
___INPUT___
These are

all


my lines
for

now.
___OUTPUT
1: These are
2: all
3: my lines
4: for
5: now.
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