{command1} {file [file2 ...]} {dest_path}
  {command2} {file_name} {dest [dest2 ...]}
  {command3} {url} {file} {word [word2 ...]} {email@address}

##</code><code>##

# assume that $_ is input line with command word removed,
# and that @tab_completions is sorted by string length:

my $i = 0;
while ( $i < @tab_completions ) {
    last if ( index( $_, $tab_completions[$i] ) == 0 );
    $i++;
}
if ( $i < @tab_completions ) {
   # $i is the element that matched, so pull that string
   # off the command line, and proceed with handling other
   # args, if any
}
else {
   # user entered a typo, perhaps?
}