...
my @colvalues1;
my @colvalues2;
my @colvalues3;
...
push @colvalues1, $allcols[0],':',$allcols[1],"\n";
push @colvalues2, $allcols[0],':',$allcols[2],"\n";
push @colvalues3, $allcols[0],':',$allcols[3],"\n";
...
open my $out1, '>>', "$colnamesxp[1].txt" or die $!; 
open my $out2, '>>', "$colnamesxp[2].txt" or die $!; 
open my $out3, '>>', "$colnamesxp[3].txt" or die $!; 
...
foreach (@colvalues1){print $out1 "$_"};
foreach (@colvalues2){print $out2 "$_"};
foreach (@colvalues3){print $out3 "$_"};