please tell me how to run the following script

Those two lines

my $parts = shift; ### how many parts to split
my @file = @ARGV;  ### the files to split
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suggest that the first argument is the number of desired parts, and any remaining arguments are the names of the files to split, i.e., for example

  ./yourscript.pl 10 file_to_split
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or maybe (if your OS doesn't support shebang lines (#!/usr/bin/perl), and you have no association set up):

  perl yourscript.pl 10 file_to_split
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