You didn't tell us, what goes wrong.

Your code is syntactical correct. But:

• Where does $n come from?
• Where does $dump_path come from?

They may contain special shell characters like &, ;, `, ', ", |, *, ?, ~, <, >, ^, (, ), [, ], {, }, $ and \.

Your CGI-script mostly does not run under the same UID as your command line script, so you should think of:

• Is your "CGI"-user able to execute /usr/local/bin/dcm_dump_file?
• Does "he" have enough rights to write into $dump_path.'PCfiles/dumpfile'?

To examine your situation, you should look at possible errors and use backticks with redirecting STDERR to STDOUT:

# see what happens
print `$your command 2>&1 > output`;
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-- Frank