hey, I assume that you were not able to match the pattern simply because you did not escape the 'special characters', namely the ":"s and the "|"s. If you can escape all the special chars, you'll probably be able to solve this yourself. Sample code that should work for multiple lines like the one you defined into $scalar, only that the "scalar list" would be put into a file which in my case is defined as scalar.txt.

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use strict;
use FileHandle;

my $scalar_file = shift @ARGV;
chomp($scalar_file);

my $scalar_handle = new FileHandle ("$scalar_file") or die "Error:Unab
+le to open file $scalar_file\n";

my $scalar;
while (defined($scalar = <$scalar_handle>)){
    print "Scalar : $scalar\n";
    $scalar =~ s"\:\:vegetable\|\|celery""g;
    print "Changed Sclalar:$scalar\n";
}

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[download]

I would however also suggest that this form of formatting could be cumbersome in complex code. Suggest you switch to arrays/hashes/csv's for data like this.
Wisdom: For every problem there is a solution that is simple, neat and wrong.