Your code:

my $target="ARGV[0]";
my @M4;
@M4 = glob('M4_${target}');
print STDOUT "the files in M4 are @M4";
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Single quotes ('...') don't interpolate variables, double quotes ("...") do. You need double quotes in the third line of your code. Also, it is possible (and preferred) to declare a variable when it is first used. Replace the second and third line in your code with

my @M4 = glob "M4_$target";
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Anno