cat file.sql | sql database -uUsername -Ppassword
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or the cheaper

sql database -uUsername -Ppassword <file.sql
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are shell commands. You need a shell to execute them.

my $shell_cmd = 'sql database -uUsername -Ppassword <file.sql';
capture_exec_combined('sh', '-c', $shell_cmd)
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If you pass a single argument to capture_exec_combined, it might implicitly invoke a shell (like system does). If so, the following would suffice:

my $shell_cmd = 'sql database -uUsername -Ppassword <file.sql';
capture_exec_combined($shell_cmd)
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