To convert a string to a shell literal, you can use String::ShellQuote's shell_quote. (If you're building a Windows command, see Win32::ShellQuote.)

use String::ShellQuote qw( shell_quote );

my $cmd = shell_quote( "prog", $_->{Sub_URL}, "(?i)Podcast", @args );
system( $cmd );
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But why invoke a shell at all?

my @cmd = ( "prog", $_->{Sub_URL}, "(?i)Podcast", @args );
system( { $cmd[0] } @cmd );
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(Ignore afoken's disparagement of shell_quote. He thinks the fact that unix and Windows are different is a defect of the module.)